The rule itself is not perfect but in practice it will probably just be the first part only: "Doublereggers are allowed to play for one regiment, if their other regiment(s) isn't/aren'tattending to NWL." because you cannot really prove who is a 'regular' member and who isn't, so I'm satisfied with it. It takes the power out of butt-hurt regiments looking for cheap technicalities to get oppositions in trouble.