It is possible to connect different "loops" to the same channel of a TIU. I use 2 TIUs, 6 channels. There was an issue with one, and while resolving that, I merely hooked its two channels to outputs of the other TIU and ran happily.
The best way to see if a transformer can handle two trains is to try it. If trains can't get enough power, or transformer starts overheating, it's two small.
Don't overlook the fact that conventional locos can be run through the remote using variable channels. Walk-around control of conventional locos is great!
