Have a read of this document on the RTI Dealer website as it provides quite a lot of helpful info.

[Link: rticorp.com]

One thing that surprised me is that it said this;

"When multiple Zigbee enabled RTI control processors will be utilized within the same Integration Designer system file, each processor must have a ZM-24 wired to the RTI COM port"

so based on that I'm not too sure if what you are wanting to do will work!

edit: you might not need to use an additional ZM-24 if you just use IR passthrough.

I would connect the two processors via a very long cable running through the house first as a test, then decide if you can run it in the walls if it all works.