  We study the phase structure of mixed-action QCD with two Wilson sea quarks
and any number of chiral valence quarks (and ghosts), starting from the chiral
lagrangian. A priori, the effective theory allows for a rich phase structure,
including a phase with a condensate made of sea and valence quarks. In such a
phase, mass eigenstates would become admixtures of sea and valence fields, and
pure-sea correlation functions would depend on the parameters of the valence
sector, in contradiction with the actual setup of mixed-action simulations.
Using that the spectrum of the chiral Dirac operator has a gap for nonzero
quark mass we prove that spontaneous symmetry breaking of the flavor symmetries
can only occur within the sea sector. This rules out a mixed condensate, and
implies restrictions on the low-energy constants of the effective theory.
