I suppose you have Vista SP1 first. If you install Win 7 Beta into another partition, it should detect Vista SP1. And when you restart your computer, it should give you a choice to boot into either one.
However, if that does not work, you can try GAG:http://gag.sourceforge.net/
It is free and it has a graphic user interface, and I think it will work with almost all Windows OSs. I tried it before, and it could do the job without any problem. Later I decided to just dual boot XP Pro and Win 7 Beta and for this I didn't really need GAG.
Edited by czhang, 06 February 2009 - 10:39 AM.