(My experience, for context: my girlfriend and I have ridden tandems quite a bit -- in the last 6 months we put in a few thousand touring miles on our tandem, and we've been riding tandems for several years now.)
There are several factors to consider when deciding who's going to be the stoker (at the back) and who's going to be the captain (at the front):
- The first and most important consideration is who's comfortable with what. If one of you is a lot more trusting (or trustworthy), that's your primary consideration.
- Another thing you should take into account is weight. The person driving the bike should ideally be the heavier person. This is just a matter of physics -- the heavier you are, the more you can affect how much the bike turns. In a pinch, it's better to have a heavier person who can more readily avoid obstacles. The better your communication and teamwork, the less important this is.
- The last consideration is Geometry. If the bike is built for a taller person in the front and a shorter person in the back (most tandems are), you should probably put the taller person in the front. Assuming you're both at around the same fitness level, this should fit nicely with the previous rule, because taller people tend to weigh more. :-)
Tandems are a whole lot of fun, but you need to go into it with the right attitude. With the wrong attitude, you'll hate it, and you'll likely make each other miserable. Here are some tips:
- Communicate more than you think is reasonable. Especially at first, you should both be talking a lot. The more you talk, the better it'll be.
- Remember you're part of a team now. If the cadence is good for you but bad for your buddy, it's bad for both of you. You'll both need to adjust some and meet in the middle.
- Be trusting. If you try to do the other person's job, you'll both be unhappy. Communicate your needs and let your buddy do their job. (This is especially important for the stoker.)
- Be understanding. If you're driving and your stoker says they're getting nervous about how fast you're taking corners, slow down. Yes, you're probably doing just fine, but riding with an unhappy stoker is going to make you slower in the end. Conversely, if you're the stoker and the cadence is messed up, understand that the captain is probably doing their best to keep the cadence in tune -- it's not as easy as you might assume!
- The stoker is in charge. This sounds weird at first, but it's important. The stoker tells the captain where to go and when to turn; the captain figures out how to get there. If the stoker says "stop", you stop. In a situation where two experienced cyclists are riding together, this balance of power is crucial for everyone to remain happy. The stoker is going to feel powerless by nature of their fixed handlebars, so the captain needs to give something back.
Tandems are fun. They go bloody fast, and you've got somebody to talk to all the time. You've got incredible amounts of power, and you can cruise at 30 mph with reasonable effort. Don't forget to stop and enjoy the moments of speedy bliss, and you'll have a great time.
A couple of additions:
- To answer the final portion of the original question, it doesn't matter who sits where in terms of strength. You're both equally connected to the drivetrain, and can both give as much power as your legs will produce.
- Don't periodically swap. Figure out who's going to be where, and practice before the ride. If you're going to be pack riding, be very very careful, as tandems aren't as agile as single bikes.
- The stoker position feels bumps a lot more than you normally feel them on a road bike. Part of this is their position directly over the rear wheel, and part of it is an inability to see the road. Suspension seat posts are a good idea for the stoker position. Most importantly, warn your stoker if you see bumps coming, and coast briefly so they can stand up.