  Mixing effects in the MSSM Higgs sector can give rise to a sizeable
interference between the neutral Higgs bosons. On the other hand, factorising a
more complicated process into production and decay parts by means of the
narrow-width approximation (NWA) simplifies the calculation. The standard NWA,
however, does not account for interference terms. Therefore, we introduce a
generalisation of the NWA (gNWA) which allows for a consistent treatment of
interference effects between nearly mass-degenerate particles. Furthermore, we
apply the gNWA at the tree and 1-loop level to an example process where the
neutral Higgs bosons $h$ and $H$ are produced in the decay of a heavy
neutralino and subsequently decay into a fermion pair. The $h-H$ propagator
mixing is found to agree well with the approximation of Breit-Wigner
propagators times finite wave-function normalisation factors, both leading to a
significant interference contribution. The factorisation of the interference
term based on on-shell matrix elements reproduces the full interference result
within a precision of better than 1% for the considered process. The gNWA also
enables the inclusion of contributions beyond the 1-loop order into the most
precise prediction.
