In their infinite wisdom, Microsoft don't let you do a simple install of both browsers side by side. I'm sure there's a very interesting technical reason behind this, but all it really does is make live difficult for developers. Tredosoft came up with a viable alternative when they packaged a couple of hacks together with an IE7 installer to make IE7 Standalone, and now Microsoft have acknowledge the grief they're giving us. They are offering a time-limited Virtual PC image containing XP SP2 with Internet Explorer 6, so that when you upgrade your main machine you can boot this VM to test with.
It's not perfect but it's a surprising solution from Microsoft and there's no denying it will work well.