The Hunter Sliver gives provoke to all slivers, the Two-Headed Sliver says that slivers can't be blocked except by two or more creatures.
Question: If you attack with one sliver and target one of the defending bears with the provoke ability, is your opponent forced to block with both bears (if they are otherwise able to do so)?
This problem is similar to one I posed in an earlier post, although in that case the question was about attackers rather than blockers.
Many a player would hastily answer to this question that "no; since the bears can't block alone, they aren't forced to do so." However, this answer would once again be incorrect.
The rule text that handles this situation is the following:
509.1c The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. If a creature can't block unless a player pays a cost, that player is not required to pay that cost, even if blocking with that creature would increase the number of requirements being obeyed.This is very similar to the rule about declaring attackers. In short, what this is saying is that a declaration of blockers that obeys less requirements than another possible declaration of blockers (which doesn't disobey any restrictions) is an illegal declaration. Only a declaration that maximizes the number of requirements (without disobeying restrictions) is a legal declaration.
In this particular example there are two possible declaration of blockers: Not declaring any blockers, and declaring both bears to block the sliver. Since the latter obeys more requirements than the former, the former is an illegal declaration of blockers. In other words, the defending player has to block the sliver with both bears.
(Note that if the defending player had only one bear, he could not block the sliver regardless of the provoke because the Two-Headed Sliver is imposing a restriction on blocking, and a restriction always trumps a requirement.)
In cases where there are several possible declarations that would obey the same amount of requirements, the defending player can choose between them. This would be the case, for example, if you attacked with two slivers and used the provoke ability of both of them. In this case the provoke abilities are forcing the defending player to block either one of two slivers, so the defending player can choose which sliver to block with the two bears.
(Incidentally, it makes no difference in this particular case whether the two provoke abilities target the same bear or different ones. While the situation is technically speaking different, the end result is the same.)
As a thinking exercise, consider what happens if the defending player had three bears, or four bears, and you attacked with two slivers and targeted two of them with their provoke abilities.
