I can only talk about the M12, but I suppose the M11 is probably similar:

To get your desired result, you have to bind each RX with a different ID number within the model - it's called "Safety Link" on the M12's bind menu. Default setting is 0 (or 1? I don't remember, but it's the same for all initially) for all model slots - if you use different numbers there (e.g. 1 for model #1 and 2 for model #2, since that is easiest to keep organized), each RX will only react if the proper model is chosen on the transmitter.