If multiple Winter Moons are on the battlefield, their effects are redundant. Each player will still be able to untap no more than one nonbasic land during their untap step.

If Winter Moon is on the battlefield while another effect restricts untapping lands or permanents in a different way, their effects are cumulative. For example, if you control both Winter Moon and Static Orb, each player will be able to untap no more than two permanents, up to one of which can be a nonbasic land, during their untap step.