Constraint Boundary Condition

Hello all,
I am constructing a 2D convection-diffusion model with two species u1 and u2.
I have a constraint boundary condition that at upper and lower boundary:
u1-u2=0 and u2-u1=0.
Does any one has some clue what can I do for this boundary condition?

Any suggestion would be greatly appreciated.
Thank you very much in advance!