Isn't it the same situation like here:
A is second order, B is first order. When 1 mole of B disappears, 2 moles of A disappear. If a reactant is zero order, his concentration doesn't affect the rate until its c reaches zero.
"What did that excess A react with if not B? If A+B-->C then dA=dB at all t."
To answer that, we should know the mechanism which is not given, so without it we are just going in a cycle.