If your multiplayer game uses the attack multiple players option, all of the opponents are defending players as the combat phase begins (C.R. 802.2). But you choose one defending player (that is, one opponent) for Stand or Fall (C.R. 802.2a) and separate only the creatures that player controls into two piles. The same is true for Two-Headed Giant, since separating creatures this way is a one-shot effect (C.R. 810.7b).

The effect says "Only creatures in the pile of that player's choice can block this turn"; therefore, creatures other than those can't block this turn, whether they're controlled by that player or by any other player. (If your multiplayer game uses the limited range of influence option, creatures controlled by players outside Stand or Fall's controller's range of influence are unaffected [C.R. 801.10].)