There is a special browser archive you can download which allows you to
"test" both IE7 and IE8 on a web page, but it was very limited, as I recall.
I tried it once, and it was just too limited, but I guess it was ok to check
and see if a single page is compatible in both versions (but that was about
all it could do). I don't think there is anyway to have both browsers
FULLY installed (i.e, as complete and fully functioning browsers), short of
(possibly) using a Virtual PC.