I have a 2 story house and have been successfully using multiple TM751's, an RR501, and a CM15A on 2 phases. In order to do so properly one needs to first plan out which signal type you are going to use to control your end devices and also what you are going to use to send the signal. My house has it's two phases primarily split with one upstairs and the other downstairs. I use both TM751's upstairs, both on different house codes, neither of which uses a house code that is used on the other phase downstairs. My CM15A and RR501 are both downstairs. My main control is my PC sending signals to the CM15A, and depending on the house code of the signal being sent, it will either send a PLC or an RF. I also have some random wireless wall switches to control some of the lights, which send the signal via RF and get received by any of my transciever modules depending on the house code. To date this has worked fairly reliably, with the only exception being the wireless wall switches whose range doesn't seem to be consistent.
I'm controlling 10 cameras, 10 light switches, 2 appliance modules, and 3 lamp modules with this setup.