From what I read on Raspberry forums, NOOBS seems to offer something like that, and also Berryboot, if I'm not mistaken.
From what I've read, it works so that one distro is the default, and if you want to boot another, you can do a one-time reboot into that other distro and do your thing there... Once rebooted, the system boots again back to "default" distro.
So, I'm thinking about having RuneAudio as default boot, and when you want to watch movies, instead of having another RasPi

Only downside is that when the machine (RasPi) is turned off, and you want to turn it on to watch a movie, you need to boot into RuneAudio first, and reboot to XBMC from there - buit that's a small price to pay for otherwise rather flexible setup.
Any ideas? is this already doable/possible?
TIA,
Denis