  We consider a class of mathematical models describing multiphysics phenomena
interacting through interfaces. On such interfaces, the traces of the fields
lie (approximately) in the range of a weighted sum of two fractional
differential operators. We use a rational function approximation to
precondition such operators. We first demonstrate the robustness of the
approximation for ordinary functions given by weighted sums of fractional
exponents. Additionally, we present more realistic examples utilizing the
proposed preconditioning techniques in interface coupling between Darcy and
Stokes equations.
