Well, if you know more or less what you're doing, installing both to one disk isn't any trickier than assigning one drive to each. I currently have no less than eight OSes (on four disks) and I haven't had any issues whatsoever. One member on this site, Saikee, even managed to install 100+ distributions to only two disks.
Still, if you can afford a second drive, why not get one anyway? Hard drives sometimes die and having a spare one - preferably with back-ups from the other one - can be a life-saver. Personally, I'd rather get two smaller ones than one big one.