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You can do multiple XOOPS installs on the same domain as long as they're in different directories and different databases. For that matter, you can even put multiple instances of XOOPS in the same database by using different table prefixes, e.g. "xoops1_", "xoops2_".
The only problem with having multiple XOOPS installs on the same domain is that cookie and session names may conflict. That's not a big problem, but it may cause login confusion if you switch between instances of XOOPS without clearing cookies and sessions.