  Polarization plays an important role in various time-domain astrophysics to
understand the magnetic fields, geometry, and environments of spatially
unresolved variable sources. In this paper we present the results of laboratory
and on-sky testing of a novel dual-beam, dual-camera optical imaging
polarimeter (MOPTOP) exploiting high sensitivity, low-noise CMOS technology and
designed to monitor variable and transient sources with low systematic errors
and high sensitivity. We present a data reduction algorithm that corrects for
sensitivity variations between the cameras on a source-by-source basis. Using
our data reduction algorithm, we show that our dual-beam, dual-camera technique
delivers the benefits of low and stable instrumental polarization ($<0.05$\%
for lab data and $<0.25$\% for on sky data) and high throughput while avoiding
the additional sky brightness and image overlap problems associated with
dual-beam, single-camera polarimeters.
