Is there conventional wisdom why bikers don't use mirrors on both handlebars in the same way as exist on a motorcycle or moped? Having both mirrors would seem to provide much better coverage which would be useful in traffic and/or riding a fast ebike.
As I've mentioned in the comments, I've got mirrors on a couple of my bikes: a Zefal Spin on the tourer and a Busch & Müller Cyclestar on the hybrid. Both are convex, and cover a wide area, and both mount on the end of the bars. They're very useful, though mainly for keeping up awareness of the situation. The B&M in particular sits quite far out and covers the lane I'm in as well as to the side where cars overtake (I'm in the UK where we ride on the left, but I'll try to make my description ambidextrous). On the rare occasions when I commute on my MTB, without a mirror, I really miss them.
But while there are cases when traffic can be coming up from behind on either side, such as when lanes split to go in different directions, you don't just need to see what's behind and to the side, you need to scan everywhere, and be seen to do so. I've also ridden in France, without moving my mirror across. I didn't find it very helpful at all having a mirror on the kerb side of my bike, even when there was traffic to that side. I'm not on an e-bike, but when I just need awareness of traffic on both sides, I'm often going faster than e-bikes legally can (here, under motor power).
Another reason: A mirror on the near (kerb) side isn't used nearly as much as on the offside; in fact they weren't required to be fitted to cars in the UK until much later. I don't ride a motorbike, but do drive a car, and there the main reason to use the mirror on the kerb side is to check before pulling in after overtaking someone, especially on a multi-lane road. You don't overtake very often on a bike, except other bikes, and everything is slower and closer together than when motor vehicles overtake each other - there's plenty of time to turn your head and check you've passed safely.
In no particular order:
- Bicyclists can see around just by turning their heads. This is much more difficult when wearing a motorcycle helmet or inside car.
- Bicycle handlebar is in a difficult position for actually looking through the mirror.
- On drop bar bike changing position would require adjusting the mirror.
- City and road bikes that would benefit most from the visibility don't have suspension, so the mirrors vibrate too much to provide clear view
- Mirrors large enough to be useful would be heavy, expensive, have huge air resistance and look ridiculous.
- All extra parts tend to rattle. Protruding parts tend to break easily.
- It seems that every time someone has a mirror on bike, will do a surprise move, possibly because they think they can see you but they don't. It's best to not look like those people.
For one mirror vs two:
- In general bikes are ridden in the rightmost lane (left in left-driving countries). The bike lanes on opposite side as seen in New York are a small minority.
I have several bikes, one of which is a recumbent. As such, its impossible to turn one's head and see behind while riding a recumbent.
Because of this I fitted some cheap bike mirrors which were rendered useless by buzzing vibrations and that they would never hold position for long.
Then I tried a helmet mirror, which was good while it was in the right place but again moved a lot. Additionally, having a big wire spike up by my temple was unnerving.
In the end I bought some expensive motorbike mirrors which reduced the buzz to almost zero. Additionally the larger mirror surface helped with view angle. Downsides here, they're big and heavy, and had to be modified to get the view angle required. And they still move over time, rotating forward or backward so still not perfect.
Of the few recumbents I've seen in use, ALL of them have at least one mirror.
Answer: A lot comes down to the requirements of the bike. Some bikes require mirrors for safety cos there are no other options. More conventional bike riders can look around easier negating some of the value of mirrors.
I tried several different handlebar mirrors over the years. They never provided a very good view -- you often had to turn the wheel sideways to see what you wanted to see, creating a hazard. They were easily knocked askew. And they were easily blocked by panniers.
An eyeglass or helmet mirror is a much better option, in most cases. The main problem is that, if you are riding low on the bars, you may need to lift your head substantially to see. But you can easily twist and turn your head to pick out a particular view of interest, without having to turn the bike to do it.
On a motorbike, you're travelling at the same speed as the other traffic, so the other vehicles are relatively stationary relative to you, their position is fairly stable in the the rear mirrors. On a bike, the motor traffic is moving much faster than and their position in the mirror changes very rapidly, it is much more difficult to perceive useful information.
One reason that was not mentioned yet:
On the road side, any accident involving the mirror is likely to be someone else's fault because they overtook too closely and it likely won't be bad, because it just scraped a car and turned the mirror in its socket. Without the mirror sticking out, people might hit your handlebar instead, which is much worse.
On the kerb side, any accident will likely be your fault, will potentially be worse if you hit a pedestrian (especially short pedestrians) and the mirror forces you to swerve farther into the road when overtaking and might even completely prevent you from overtaking in tight spaces.
I ride/rode several recumbent bikes and trikes as well as upright bikes.
On both trikes I happen to have mirrors on both ends of the handle bars.
On one 'bent bike I had a single mirror placed on the over seat handle bars, on the left side, looking around my body on the left side.
On the other 'bent bike I do not have a mirror but use a mirror which clips onto my glasses.
One trike came with mirrors on both sides and as the one on the left tends to move around more (it gets hit when I park the bike at work,) I use the right side mirror at times to see the cars behind me. And that right side mirror is useful for cyclists close behind me.
On the other trike I had the second mirror installed as I hold the end of the handle bar which sticks up and the different feel did drive me crazy. I had a travel to England coming shortly after and did appreciate the right hand mirror, although I can and do see behind me with either mirror. These days that trike is stored and only gotten out when the roads are iced over, the mirrors failing at the connectors to the handle bars and I am not interested in replacing them, as the glasses mounted mirror works as well.
Would I install two mirrors on any of my bikes or trikes (outside where I hold onto the connectors,) no. While having a second mirror is useful, it is not worth it most of the time.
Adjusting the one mirror on the left takes little time and it is enough to cycle safely.
The most important thing is to find a mirror that works for you. It does not need to be an expensive one. The one I have on the over seat handle bar was the cheapest of the big bike store and it worked for a long time without any problems. It sits more to the middle of the bar, in a position where it is not banged about.
One of a set of plastic mirrors (I had two on that trike) got 'broken' the first day when someone parking next to me did not see it and pushed it too much. It could be used again but never was as sturdy as it should have been.
One of the other answers mentions that bike mirrors did not work for them, due to them shaking. I found that more depending on the road than on the mirror. If your road is not having a flat top, all mirrors will be less than optimal. If your mirrors do not work as you need them to, invest in new ones. Talk with other mirror users, if you know them.
Having owned three kind of mirrors to clip on glasses (or helmets) I am only happy with one. But I am so happy with that one that I will happily use it on all bikes I ride, even the uprights where I had never considered a mirror before.
You mention speed as a reason to use two mirrors. On one 'bent bike I repeatedly overtake e-bikes, on bike lanes and small roads with little or no cars. On that bike I use the glasses mirror and I do never worry about having one on the other side of my head. Just like on an upright bike without mirror, you know your speed and you can guess the speed of the other(s) when you have passed them. One glance in the mirror when you feel it should be safe to go back to the side of the road is enough. A mirror on the left of the handlebar should also give enough of a view behind you that you can safely judge the moment to go back to the curb.
The difference with motorized traffic is that you will never have a steady stream of fast traffic to your right (or left if in a left side drive country) like you have in a middle or fast lane of a motorway. And even in that kind of traffic the laws of many countries allow you to only have one wing mirror.