Hmm, I knew of the double.. didn't know it had any benefit for marshal though.

It's in Dragon Magic, btw.

This feat is different, you don't have to be 12th level for this, you can project 2 nondraconic auras (even the marshal needs to switch out major auras for draconic auras to get the benefit of the double draconic aura), and this stacks with double draconic aura.