Epochalyptik says... Accepted answer #2
In all zones except on the battlefield while transformed, Elbrus, the Binding Blade Flip has only the properties of its front face. Withengar Unbound Flip doesn't exist until Elbrus is transformed. Therefore, the card is not a legendary creature in its default state, and is not a legal commander according to the official rules.
However, you are free to ask your playgroup whether they would consider a house rule that allows you to play with it anyway.
TheRedMage says... #1
No. The front side of the card needs to be a legendary creature.
June 30, 2015 1:31 p.m.