2
It won't be a problem, in fact I've run 5 at once. All you need to do is create a new directory for each XOOPS root. Create a database fr each installation, or if you only have one DB, create a different DB table prefix, like xoops01, xoops02, etc. Either way will work just fine.
Becareful not to overwrite existing tables with the same prefix if your using the same DB. Thats the only thing you need to be careful about.