I bought the USB multiplexer and the XKeys pro and they do NOT play well together. I got them working, but it was unreliable. I talked to both XKeys and Vetra and they said the PC's need to be exactly the same in order for something like that to work correctly. My setups were very similar, but not always the same, so it might explain why I was having issues.

I ended up buying the PS2 8 port and another XKeys and never looked back.