If you have two Xathrid Demons, both abilities will trigger at the beginning of your upkeep. When the first Demon’s ability resolves, you can sacrifice the second Demon to satisfy the requirement (“Sacrifice a creature other than Xathrid Demon” really means “Sacrifice a creature other than this creature”). When the second Demon’s ability resolves, you must sacrifice another creature. If you can’t (because the first Demon has somehow left the battlefield and you control no other creatures), you lose 7 life, regardless of whether the second Demon is still on the battlefield at this time.