My IR receiver becomes erratic after one motor in the circuit starts spinning
My first suggestion here is to place at least 100nF ceramic disc capacitors across the motors to reduce the EMI emissions. You could also place additional 1-10µF capacitors in parallel with the ceramic capacitors.
If that doesn't help, you could add small inductors (one to the positive, and one to the negative side to each motor) and the 100nF caps towards the controller side (4 inductors and 2 capacitors in total; basically an EMI filter).  
You should also add at least a 100nF capacitor across the supply lines right next to the IR receiver if there isn't one already. Same with the micro-controller supply pins.  
You also need at least 100nF across the battery terminals or the positive side of the motors and the ground. You can go as high as 100µF or even 220µF here if necessary.  It's possible that the motors are causing voltage "dips" when starting and thus confusing either the IR, the MCU, or both.  
Finally, the grounds need to be separate and ONLY come together next to the power supply, to prevent ground loops. If the ground line is very thick/wide or you have a good ground plane, this may not be necessary.