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#11
From a legal point of view, you are only allowed to install one. In your case you will legally need to purchase 2 disc/licenses. I know that means you will have 4 discs (2 x86 and 2 x64) but you will be legal.
As for the clean install from disk, I would assume so, it will look for a previously installed OS and "upgrade it". For many of us, that means we can just "upgrade" our Windows 7 beta (I hope).
I am planing to clean off all 3 of my OS on my desktop (XP, Vista x64 and W7 beta), reset the partitions to the full drives, and going only to Windows 7 x64 on the main drive and Ubuntu 9.04 on the secondary drive. The other computers in my house will stay with the XP that is installed.... until I get another W7 license or two to update a few of them.