Not sure why you'd want to do that. You can mount remote drives onto the Humax if you're wanting to play movies using the Humax off a drive on another machine on your network, but unlike XBMC, which will play just about anything you throw at it, the Humax is limited with regard to what type of video file - you can't play .mkvs on it for example.
The easiest way to have an XBMC-like user interface in two places is to run two instances of XBMC and point them to the same sources, but of course you need two computers to do that. I have tried a raspberry pi as a cheap alternative but it's paint-dryingly slow even with all the tweaks. Laptops (even netbooks), Android tablets and iPads work very well. You can use uPnP to share libraries between XBMC instances (video here
). You can also set up a mySQL server to synchronise all your XBMC nodes (so you can watch half a movie downstairs and carry on from the same point upstairs for example).
Best way to find out more is to visit the XBMC user forum
or check out the Wiki
Apologies to the mods if this is going off-topic.