The main problem will be the output power of the boost converter. If each of the motors draws less than 500 mA when stalled, you should be OK. You may want to add a large electrolytic capacitor on the input to the Arduino, as well as small ceramic capacitors across the motors, to make sure that spikes don't make it into the Arduino power supply.
If your motors draw more than 500 mA each when stalled, your Arduino will likely reset or misbehave when load goes up -- which includes right at start from stand-still.