Rulings

Ember Beast can’t attack or block unless another creature is also assigned to attack or block at the same time. Notably, two Ember Beasts can attack or block together.
Once Ember Beast has been declared as an attacker or blocker, it doesn’t matter what happens to the other creature(s).
Other creatures assigned to attack alongside Ember Beast don’t have to attack the same player or planeswalker. Other creatures assigned to block alongside Ember Beast don’t have to block the same creature as Ember Beast.