  We investigate the advantages of using two independent, linear detectors for
continuous quantum measurement. For single-shot quantum measurement, the
measurement is maximally efficient if the detectors are twins. For weak
continuous measurement, cross-correlations allow a violation of the
Korotkov-Averin bound for the detector's signal-to-noise ratio. A vanishing
noise background provides a nontrivial test of ideal independent quantum
detectors. We further investigate the correlations of non-commuting operators,
and consider possible deviations from the independent detector model for
mesoscopic conductors coupled by the screened Coulomb interaction.
