I'm not 100% on this now but I'm pretty sure that you cant have completely different installs of the two browsers. IE7 will upgrade certain components within the OS which will then be used by both IE6 and IE7. I tried to do it on my work machine and it ran fine until we realised that IE6 wasnt actually behaving the way it should and was using those components from the new version.
You'll notice the second you install IE7 that the fonts and overall look of websites improves drastically. If you then go and use IE6 you will see the same improvement in that browser too. I'm not sure if some smart bloke has figure out a different hack for it by now though.
If that sounds like what you want then you could
run through this guide.