If a Modal Double-Faced card has a legendary creature on its front face, the card can be your commander. You may cast either face from the command zone. Command tax is applicable to the card itself, regardless of which face you cast. For example, if you cast Valki, God of Lies and it then dies, casting Tibalt, Cosmic Impostor will cost 7BR and recasting Valki will cost 3B. If both faces are creatures, both faces will deal commander damage in combat.