If your tank is developed enough, a couple days without food shouldn't hurt them. They'll just clean the algea from the tank walls and bottom. If it's a new tank, I'd be careful. Bettas don't usually bother algea wafers, so if you have those (and your betta doesn't snack on them) then you can use those.
If the bettas do munch on the sinking food, and the tank is new, you may have to move them or the betta until the fasting is over.