The risk you run is blowing the voltage regulator in one or both of the motors. This is why you don't connect them together while running.
They arent designed for this, and if you do pop one, you'll play hell getting it fixed under warranty.
One of the reasons for dual engines is for redundancy. If they are wired together on the positive wire, you have lost that redundancy. How do you run on a good engine after the other one has just fried all its wires? You cant when their wired together.
Invest in another switch. It may save your life someday.
