6 wheel holonomic drive feasibility

[ekapalka]

My team has been tentatively considering designing a 6 wheel drive using two sets of mecanum wheels with a centrally located set of omni wheels (a little like the one on this thread) to give the robot pushing power. When I say tentatively, I mean like we may-or-may-not be designing our next-gen chassis with holes for two central gearboxes (which could also be used in the event that we give up on holonomic and resort to skid steering). What I was wondering is how to control the center wheels so as to not conflict with the mecanum wheels while still providing pushing power. I've considered having each side sync with the speed of the mecanum wheels if and only if that particular side is turning in unison, but that would cause them to drag when strafing at an angle or when rotating while strafing. I've also considered setting them to match the averaged RPM of all four wheels, but I don't think that would work either. One other thing that came up in discussion was to have a non-powered wheel to measure the absolute speed forward of backwards and sync the middle wheels with that, but that wouldn't work if you're in a pushing match and not moving at all. What do you think? Is the only way to do this really to use a ball differential?