You can have different versions i.e. v9, v10, v11 etc. on the same pc as they all install as separate programs. You cannot, however, have more than one of each version (e.g. two copies of v11), on the same pc. If you want to have more than one site in the same version, then you need to buy either a multi-site version of the software or an extra license and go to 'File | Sites | License new site'.