  We study the renormalization of the gap of an s-wave superconductor in the
presence of two magnetic impurities. For weakly bound Shiba states, we
analytically calculate the part of the gap renormalization that is sensitive to
the relative orientation of the two impurity spins. For impurities with a
strong exchange coupling to the conduction electrons, we solve the gap equation
self-consistently by numerics and find that the sub-gap Shiba state turns into
a supra-gap Andreev state when the local gap parameter changes sign under the
impurities.
