Am not a very big fan of carrying repair kit around. This is it, no room for surprises and mistakes, I repaired multi punctures before they happened. My front wheel had a new innertube already. But my tire is old, I don't trust it very much. Carried my old unrepairable tube to my mech and had it installed together with new one as an additional layer of protection. Same wheel, say future proofed. Actually I think the front wheel rolls a bit better this time, also it doesn't needed as much PSI as when it had single tube inside. I also have a old spare tire laying around I might use in the future. Could I be making a mistake? What could possibly go wrong.
Am not a very big fan of carrying repair kit around.
Neither am I since I hate roadside repairs. Since it's unfeasible to have as many CO2 cartridges as I have patches in my patch kit, I have to carry a mini pump anyway and to save weight, I don't use CO2. So pumping up the tire after repair with a mini pump is a major chore.
But since punctures can and do happen, I still have to carry a repair kit around.
But my tire is old, I don't trust it very much.
If you want the level of reliability you get with automobile tires, you need to have a tire with as much rubber as in automobiles. I don't believe there is such a tire. Maybe Schwalbe Marathon Plus could have half the thickness of rubber (not sure, haven't measured, I don't use Marathon Plus), but since it has only half the thickness, you still get punctures.
I few years ago collected a sample of the type of gravel they distribute on pavement to prevent pedestrians falling on ice. It had about 14mm long sharp rock fragments that could puncture a tire unless it has more than 14mm of material.
The only defense against those is a very very thick tire armor or at least 15mm rubber in the tire. Any other kind of defense simply won't work. Kevlar or similar belt doesn't help.
They do sell those kinds of tire armors: Tannus Armor. Unfortunately, they result in squiggly cornering performance and horribly high rolling resistance. The rolling resistance in normal conditions loses you probably about 100 times the time you would lose when simply repairing the punctures as they happen.
However, Tannus Armor has its place. In areas with snow and ice, when they start to distribute that gravel, the frequency of punctures increases maybe 50-fold and due to cold wet weather, the annoyance of having to do repairs increases greatly. So in those conditions and those conditions only, Tannus Armor should be used.
But in agreeable biking conditions, don't use a Tannus Armor. It's a mistake. Repair punctures as they happen.
Any sold tire (unless you are considering airless tires which are even worse than Tannus Armor) is vulnerable to punctures. Any non-tubeless tire is vulnerable to pinch flats too: in fact I have had more pinch flats than punctures recently. If you want to prevent pinch flats, pick a very wide tire, at least 45mm. But low-rolling-resistance tires are not available for such sizes, so you lose in rolling resistance too. And unless you use Tannus Armor too, you are still vulnerable to punctures.
Carried my old unrepairable tube to my mech and had it installed together with new one as an additional layer of protection.
This doesn't work as the old tube is not as thick as Tannus Armor. In fact, the material in a tube is punctured very easily.
Could I be making a mistake? What could possibly go wrong.
If you omit carrying the repair kit around, then you know what could go wrong!
Humans have very low power to weight ratio compared to cars. It might be possible to make a bike tire that is practically impervious to road hazards. However, it would be extremely slow, heavy, and uncomfortable - the latter because the extremely thick casing would not deflect to cushion road bumps.
If you want high puncture resistance, tires like the Schwalbe Marathon and Continental Gatorskin are known to be pretty good. They have kevlar strips added under the tread. However, many people feel they're heavy, slow, and uncomfortable - that said, those folks may be used to high performance tires. You can also buy tire liners to add to an existing tire. However, they can be fiddly to install.
If you don't want this option, tubeless may be an option because the required sealant will plug punctures. The caveat is that it tends to seal better at lower pressures. With road size tires of up to 28mm, I feel like the advantages aren't clear yet.
It isn't clear exactly what you did. If you used an old tube as a makeshift tire liner, this seems like it might help, but you're taking up a lot of space in the rim cavity. It seems to me that the risk would be that the tire isn't fully supported from the inside by the inflated tube. It could squirm against the extra tube, which could be a problem in a corner.
Another thing to think about is that old tires may get more punctures. After all, the tread is thinner, so there's less material for a glass shard to push through. The purpose of the tire is to make the bicycle roll, balancing puncture resistance, grip, and rolling resistance, all of which is to make you enjoy riding more. Absolutely maximizing tire lifespan is not the end goal of the tire.
I do not think that an old tube over the new one would protect much at all. If a thorn manages to go through the tyre, it will puncture one tube layer almost as easily as two tube layers. You can buy a well puncture-protected tyre, you can avoid using too old tyres by changing your tyres often enough but I do not think another layer of butyl inner tube will achieve that much. It certainly won't make it unnecessary to carry a repair kit and a mini pump, you should always have those.
Dedicated tyre liners, mentioned by Weiwen will certainly be much more effective than an old inner tube.
As Weiwen notes, you could consider tubeless, I am using it happily on my gravel bike but I also still use inner tubes on my old mountain bike or on my road bike. Some fill their inner-tubes with slime (a kind of sealant), with mixed success.
There are also airless tyres, but they really worsen the ride quality and the speed of the bicycle. Not even street rental bikes in my city would use those.