It would be interesting to connect an oscilloscope to the connection between the motors, to see what happens.
For the underwater robot we're working on with surface batteries, we decided to use a 48v power system, to keep the tether size and tether voltage drop percentage to a minimum. My brother designed and built a couple 4 channel speed controllers that control 12v motors, but give them 48v for up to 25% duty cycle each. It seems to
work fine....maybe you could do the same thing with one CIM per victor, running at 24v, or just try them at 12v and see how they work.
I didn't look at the specs for the 885, but does it give a minimum voltage? The Banebots speed controllers are spec'd for 6v to 24v, we used some last year at 14.4v.