  Let G and H be two locally compact groups acting on a C*-algebra A by
commuting actions. We construct an action on the crossed product AXG out of a
unitary 2-cocycle u and the action of H on A. For A commutative, and free and
proper actions of G and H, we show that if the roles of these two actions are
reversed, and u is replaced by u*, then the corresponding generalized
fixed-point algebras, in the sense of Rieffel, are strong-Morita equivalent. We
apply this result to the computation of the K-theory of quantum Heisenberg
manifolds.
