MorganS:
If you don't have experience then you must design with calculations. You only need multiplication and division. It's not a differential equation.The power is limited by the controller. Two motors will have the same power as one. And they won't share that power equally. One will get more than the other, and not consistently.
How to do those calculations? I've tried to dig to motor torque theory, but It's a bit intimidating for a noob like me.
Well, If You say they will not get even power than perhaps I should trust You 
Is it valid to reuse same output from Arduino and split them to 2 controlers? I am afraid I will run out of pins 
I will post some 3D desigh soon so we can discuss belt/gear and two motors issue.
Thank You for Your help, will be back with 3D desighns soon.