If you have the TM751's isolated, it should work great. By that, I mean if their power line signals don't collide. That would happen if they were on the same leg of your home wiring, for example. As hawk1 stated, any RF signal received by a TM751 would be transmit on the powerline at that time. So if two TM751's received an RF signal, they would transmit it on the powerline, and if they are on the same phase, the signals would collide and may not get sent. If only one receives the RF command, or if they were on different electrical phases, this could be avoided.
Another option would be to set one on house code A, one on B, one on C - OR use the PAT01, or RR501 transceivers, which are known to be more "polite".