It's completely possible to install multiple versions of Notes on the same machine. I have 7 and 9 on my main workstation.
At the very least, you absolutely need to have separate directories for the binaries, and for the data, something along the lines of
c:\program files\ibm\notes8\... and c:\data\notes8 on one hand and
c:\program files\ibm\notes9\... and c:\data\notes9 on the other hand.
Then the tricky part is sometimes with the Windows registry. It seems that the installer first checks the registry and when it sees the existing installation, treats the new install as an upgrade. You can first weed out the registry of everything Notes related. Notes does not really need those. It's a bit tedious, but it works.
Another approach is to treat the new install as a *portable* installation, making it independant of the existing installation and of the registry. Does not need to be on an actual flash drive, but as above make sure to use independant and watertight sets of directories.
A quick googling yields those two results :
http://it.stonybrook.edu/help/kb/portable-notes-on-a-usb-flash-drive
http://julianoclaiton.blogspot.fr/2011/05/lotus-notes-client-portable-usb.html
Using a method similar to the second one, I was once even able to install a second version in userland (%userprofile/mynotes/...). This, of course, depends on the policies enforced by your corporate IT. Depending if said corporate IT is cooperative or not, you may want to talk to them : they may have tools to help you tweak the registry and around Windows permissions.