Xantech blocks work fine with the UIRT's output jack. I cut the emitter off a single Xantech emitter, stripped the ends, and wired one into gnd and the other into signal in - leave the +12V empty.
edit: If you need to control 2 cable boxes, you will need a UIRT/MD for each. If you only have one system to control both boxes, you will need an IR router like the Xantech RT8 and have to add a command in the media scenario for each box to select the correct port. Even though the RT8 would work, it would be SIGNIFICANTLY less complicated and almost the same price to just add a MD.