  Heavy meson pairs produced in the decays of heavy quarkonium resonances at e+
e- machines (beauty and tau-charm factories) have the useful property that the
two mesons are in the CP-correlated states. By tagging one of the mesons as a
CP eigenstate, a lifetime difference of heavy neutral meson mass eigenstates
width difference may be determined by measuring the leptonic branching ratio of
the other meson. We discuss the use of this and related methods both in the
case where time dependent mixing is small and when it is significant. We
consider the impact of possible CP-violating effects and present the complete
results for CP-entangled decay rates with CP-violation taken into account.
