Mario, do you have matching firmware on the extenders? And also, please be aware that the Prontos communicate with the extender using UDP and sequence numbers. There is a handshake up front when Pronto wants to use the extender. If the extender is busy, it should report such.

However, based on the design of the communication protocol between TSU and RFX, the RFX9600 and RFX9400 were never really intended to support more than a single Pronto. That's the only way I've ever run the setup with a single exception. My lighting controller is on a single RFX but I can safely say that I have never used 2 remotes on same extender at one time as I'm either in the living room or the bedroom.