Yeah, you can dual boot. I'm sure there a quite a few guides out there. But easiest is probably to install debian on another partition and then install SteamOS via the PPAs.

In regards to porting to other OSes with smaller market share. I think Square should release a MacOS client. I think that SteamOS will end up having greater reach, however, as OEMs begin to sell SteamOS "consoles". I think Square would be well served to port early and be one of the first AAA MMO titles on SteamOS.