It is possible to run both Plus! versions but there's a few things:
- If you reinstall
Plus!3.63 (note this is a newer version than the one you mention) when you import a soundpack (by doubleclicking on the downloaded pack or when choosing open/run on downloading a pack from mpsounds.net) you will get a message notifying you that Plus! must be running first before you can import. If you import a downloaded soundpack through the Sound Library itself you won't have this problem.
Solution: reinstall Plus!Live again and start Plus!3 manually through taskmanager or in startupfolder...
do not uninstall Plus!3. If you didn't uninstall Plus!3 prior to installing Plus!Live, wich i recommend not to do anyway since Plus!Live will import some settings from Plus!3 on install, then reinstall ain't necessary and you can start Plus!3 from taskmanager.
- When you exit Windows Messenger (with Plus!3 running), Windows Live Messenger will sign out too.
- The Personal Status box will be triggered on away for both if you run them at the same time.
Wondering about why you want to run both Windows Messenger and Windows Live Messenger: there's polygamy in Plus!Live (Plus >Preferences > Messenger) so if it's for logging in multiple accounts then you won't need to.
Edit: Reason why Plus!3 won't be loaded when you start Windows Messenger after you install Plus!Live is the fact that Plus!Live disables Plus!3 on install.