Cross diffusion coefficients in coefficient form PDE interfaces

Hello,

I'm curently working on some numerical simulations with a two-variable PDE system shown in the attachment.

The last terms in both functions are the second order Taylor expansion terms. A and B are the diffusion coefficients.

Since the diffusion terms here are cross diffusion, I don't know how to implement the coefficients into the diffusion coefficient part of the PDE interfaces. What should I input into the four boxes under Diffusion Coefficient?