  We investigate the problem of coordinating two robot arms to solve
non-monotone tabletop multi-object rearrangement tasks. In a non-monotone
rearrangement task, complex object-object dependencies exist that require
moving some objects multiple times to solve an instance. In working with two
arms in a large workspace, some objects must be handed off between the robots,
which further complicates the planning process. For the challenging dual-arm
tabletop rearrangement problem, we develop effective task planning algorithms
for scheduling the pick-n-place sequence that can be properly distributed
between the two arms. We show that, even without using a sophisticated motion
planner, our method achieves significant time savings in comparison to greedy
approaches and naive parallelization of single-robot plans.
