  Critical surfaces are defined by Bachman as topological index 2 surfaces,
generalizing incompressible surfaces and strongly irreducible surfaces. In this
paper we give a condition to obtain critical Heegaard surfaces by amalgamation.
As a special case, we obtain critical Heegaard surfaces by boundary
stabilization. It gives critical Heegaard surfaces of non-minimal genus, for
3-manifolds which do not admit distinct Heegaard splittings (up to isotopy).
