  A reduction formula for the branching coefficients of tensor products of
representations and more generally restrictions of representations of a
semisimple group to a semisimple subgroup is proved in work by Knutson-Tao and
Derksen-Weyman. This formula holds when the highest weights of the
representations belong to a codimension 1 face of the Horn cone, which by work
by Ressayre corresponds to a Littlewood-Richardson coefficient equal to 1. We
prove a similar reduction formula when this coefficient is equal to 2, and show
some properties of the class of the branch divisor corresponding to a
generically finite morphism of degree 2 naturally defined in this context.
