Sinuous Predator can't be blocked by more than one creature.
For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).
If Sinuous Predator gains menace, it can’t be blocked by only one creature and it can’t be blocked by more than one creature, so it simply can’t be blocked.