Two mags taped together like this are done this way to make it very easy to swap mags quickly as the mag you're removing and the mag you're adding are attached and so you only have to reach for another after expending 2 mags. So when you run out of ammo in the first mag, you grab it, release it, flip it, insert it and done rather than having to reach down to grab a new magazine. But it doesn't actually extend your ammo capacity before reloading. This really only increases reload speed slightly instead of adding ammo capacity. And unless you have multiple sets of mags taped like this, it's really just good for one time and then you're reaching for individuals unless you tape another set up between fights.
As far as realism in extended mags, this isn't that unrealistic. Many weapons have extended mags available for them and drum mags are a thing for certain weapons as well. Maybe it's a little strange with certain weapons we have in the game - I don't really know what real life versions of these weapons do or do not have extended mag sizes - but it isn't all that unrealistic. I don't think there's anything wrong with what we have for ammo capacity mods right now.
I will say, in terms of the rest of the conversation here, that two mags taped together would likely be used in such a way that the first mag would be run empty before reloading, or at least close to empty. After that, it would probably be swapped out at specific points in a battle where you know you're going to need a lot of ammo and you are low enough in the mag that it just makes sense to swap before the next shootout. Depending on what you're expecting, you might be swapping at 50% or not until 25% or some other amount and it also depends on what's going on. If you're in the middle of a fight, you're more likely to use everything before swapping because the time to swap won't be any different if you empty the mag versus swap early. It between fights that you're likely to swap a mag that isn't completely empty with one that is full. So, in that, I do tend to reload at times that wouldn't be smart in a real fight. I often find myself reloading a 44 after only 2-3 shots and sometimes have zombies coming at me as I do so. That wouldn't be the smartest option in real life. The game reminds me of that when I hit reload only to have a zombie start eating me before the reload completes.