I'm just brainstorming here. It'd be a good DIY project for those with the tools and means.
For a multiple flask stirrer, it might work to have one variable-speed motor, two 4-inch wheels (same size, lying on its side), and a belt connecting the two wheels (so the wheels move in sync). Attach your multiple flask stir plate (wooden or metal board)with its own hub and axle to the 12 o'clock position of each wheel. As the wheels turn (in sync), the board moves in a gentle circle (without any pitch, yaw, or roll).
I'm sure the parts could be found at Harbor Freight or McMaster Carr.