Recent years have seen increasing attention to the subtle effects on
intracellular transport caused when multiple molecular motors bind to
a common cargo. We develop and examine a coarse-grained model which
resolves the spatial configuration as well as the thermal fluctuations
of the molecular motors and the cargo. This intermediate model can
accept as inputs either common experimental quantities or the
effective single-motor transport characterizations obtained through
systematic analysis of detailed molecular motor models. Through
stochastic asymptotic reductions, we derive the effective transport
properties of the multiple-motor-cargo complex, and provide analytical
explanations for why a cargo bound to two molecular motors moves more
slowly at low applied forces but more rapidly at high applied forces
than a cargo bound to a single molecular motor. We also discuss how
our theoretical framework can help connect in vitro data with in vivo
behavior.