  We consider biorthogonal polynomials that arise in the study of a
generalization of two--matrix Hermitian models with two polynomial potentials
V_1(x), V_2(y) of any degree, with arbitrary complex coefficients. Finite
consecutive subsequences of biorthogonal polynomials (`windows'), of lengths
equal to the degrees of the potentials, satisfy systems of ODE's with
polynomial coefficients as well as PDE's (deformation equations) with respect
to the coefficients of the potentials and recursion relations connecting
consecutive windows. A compatible sequence of fundamental systems of solutions
is constructed for these equations. The (Stokes) sectorial asymptotics of these
fundamental systems are derived through saddle-point integration and the
Riemann-Hilbert problem characterizing the differential equations is deduced.
