Hello Turnskin, and welcome to Seven Forums.
You will not be able to activate both Windows 7 installations with the same product key number. If you have a second product key to use to activate the second installation of Windows 7, then you could install it like this.
You can use the tutorial below as a guide for how, but just substitute Vista for the other Windows 7 instead. Dual Boot Installation with Windows 7 and Vista
Hope this helps,