  We derive the high frequency limit of the Helmholtz equation with source term
when the source is the sum of two point sources. We study it in terms of Wigner
measures (quadratic observables). We prove that the Wigner measure associated
with the solution satisfies a Liouville equation with, as source term, the sum
of the source terms that would be created by each of the two point sources
taken separately. The first step, and main difficulty, in our study is the
obtention of uniform estimates on the solution. Then, from these bounds, we
derive the source term in the Liouville equation together with the radiation
condition at infinity satisfied by the Wigner measure.
