Hi, basically there would be no bigger problem to create a central metro station, but there are some drawbacks:
1. It would require 2 slots for stations in the trackst.dat - one for the right and one for the left side. This means there would be no more space for any other stations, for example the existing metro station or the elevated station from the Berlin pack.
2. Graphically, it wouldn't look great, as the distance for the "roof" would be too small - the central platform, made of two stations, would be very narrow. That's why I actually chose to have a Loco-style metro station with platforms on both sides. Not realistic, but for Loco it's the easiest and simplest solution.
I'm not saying that it's a bad idea, I might try in the future, it's just that Loco has some limitations, especially the number of stations, that make it hard to go around and build new stuff. As for the Rusitch you sent me an e-mail about, I might try later this year, but I'm a bit busy these days at work and it doesn't look like it'll change soon.
Anyway thanks for your comments, I'll keep them in mind and might translate them to the game if I find enough time.