[Rutherford]

Isn't it the same situation like here:
2A+B C
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.

[Borek]

Why do you ignore stoichiometry? Reaction has to follow both the stoichiometry and laws of kinetics, it can't follow one of them but ignore the other.

If the reaction equation is A + B C no matter what kinetics is, 1 mole of A always react with 1 mole of B. If 1 mole of A disappears, 1 of mole of B disappears. There is no way that something else can happen, no matter what is the mechanism.

[Rutherford]

I see now that I got confused, because I saw the different plots for different order reactants. Now I understood that the concentration change of B depends on the concentration change of A (they must have the same plot regardless of the order).