Start thinking about your needs and which devices would support it.
The simplest thing, would be to build another HTPC exactly the same as your current one and repeat - if you don't have a TV for it to drive then that can be a secondary purchase. Essentially your HTPC2 with Openelec will just be a NAS until you hook it up to another display.
This actually raises an interesting situation that a LOT of guys are experiencing all over the place - How do I synchronize media between the devices, can it run in master slave config between XBMC instances, etc.
Luckily, the guys at Openelec are working on this problem and intend on offering either a master / slave release differentiation and / or a service that will enable a mysql decentralized master library to be hosted, and all other xbmc instances to come and go as they please.
Whut?
If you get a 2nd HP Proliant you don't need to buy a second TV dude...lol
Just connect both HP's to the same network via CAT5e cable, and set each XBMC to use the other's media (as well as it's own of course)
(If you do have 2 TV's though, both TV's will be able to access both HP's Media, meaning Everything)
I really have no idea why this should be a problem, what master/slave config you are talking about, why you would need mysql, or "synchronizing" between HTPCs