Good Evening,
Your problem sounds relative simple, but complicated to solve. It appears you are having collision problems on the power line, not phase problems as each transceiver works when it is the only one active. Try to separate the transceivers as much as possible. If you get them separated far enough things will work OK when at the ends but not when in the middle area, this confirms the collision problem but is not a conclusive test The best solution would be to use one and save the other as a spare if one will cover the distance you need.
What is this? There is no arbitration in the system so when a transceiver receives a signal it then blindly transmitters it on the power line. You have two transceivers doing this. The internal propagation delays in the two transceivers are different consequently they will put the same signal on the power line but at a slightly different time. The more transceivers you have the worse it gets. One could be sending a 1 the other a zero. Which is dominating? I don’t know but the results are the same the code gets scrambled i.e. a 0 gets changed to a 1 or a 1 to a zero times several bits. Then the code the module sees is not valid so they do not work.
Your best solution if one unit does not work and you have the resources then modify the antenna system on one transceiver to make it sensitive enough for your whole home. Another solution would be to have one of the transceivers delay putting the signal on the power line for a few milliseconds, this would have them appear sequentially rather then collide. I do not have insight to the circuits so I do not know if this is actually feasible.
Have Fun,
Gil Shultz