When you say another hdd do you mean another computer? Or like you have 1 computer and 2 hdd's with the OS?
I'm gonna assume you mean the second idea and it looks interesting. I see no reason for it not to work. Reformat both drives. Put one in, install vista. Take it out, put the other one in, and install vista. That's a really good way to back up. When one thing fails, just pop in the other hdd
Hm, well, I can't really format my current drive, since I don't have enough to back it up, but the second drive will be a fresh install. Can't I just use my disk and key, and install it on the second drive? I just don't want to activate it on the second drive, then the first drive would get messed up or something.