Here in the US, no form of multiple marriage is legal. That, however, in no fashion precludes religious or non-religious binding ceremonies with multiple people. There's just no legal recognition of those ties.
Thus while Pagans can have handfastings with more than one person, only the legal spouse has the legal rights associated with marriage. Any other spouse, though handfasted, is not recognized by the law. It's not a matter of one partner having more spousal rights than another--one has all spousal rights and the other has none.