OK Brett says the direct link will not work between two XP8's with one of them being the Zm24 only.

however I have devised and Idea that might actually work. This involves creating two sets of system macros, one on each XP-8
ID lets me set up the scenario this way but I do not have two XP8's to test this out so I don't know for sure if this will work, but it goes like this.

Create a system macro on the upstairs XP8 with a command you want to execute from down stairs. give it a name like TV power on (B) put your IR or serial command in here

Now on the downstairs XP8 create a system macro and call that TV power on (A) Drag System Macro macro (B) into System macro (A).

On your Z remote make a stand alone button and create a macro on that button that includes system macro (A)
This in theory should trigger (A) which in turn asks the first XP8 to do the core of triggering (B). Hopefully we can bypass the Z limitation this way.

Give this a try and let me know if it works :-)