We tried connecting the 2 motors to the axle with chains/ sprockets and that did not seem to work well…chain slipped- we added standoffs for reducing slack which yielded mixed results.
Any ideas on what is an efficient way to do this? Again - both motors need to be on one side!
Send pictures or further elaborate. It is very hard to think and visualize your problem as it might not directly relate to what you said.
Use gears. If supported on both sides of the axel, there should be no slippage.