How to reduce motor electrical noise?
Firstly look how I've modified the power and ground leads below (in red): -



It's important to star-point different circuit blocks back to their respective batteries in order to minimize loop current emissions. The important one is the 12V local to the L293D chip. Talkingof which, be sure to use the version with a "D" on the end because that has internal flyback diodes and without these you'll be producing voltage spikes that could interfere with tesla coils (that was a joke that last bit).

Next are decoupling capacitors across each motor - If you are using PWM control you need to be a bit cautious with high values as these will suck energy from the pulse width modulation. Try 10nF across each motor and if necessary try a 10uH inductor in series with each motor with a bigger cap (like 100nF) still directly across the motor.

The problem could be also solved by making the whole of the 0V system a local earth plane - there may still be currents flowing in your current set-up that cause the HT12D to glitch and lose control.

Make sure you have all the major electronic components decoupled with caps too. Maybe 10uF and 10nF across the HT12 and L293D and receiver. HT12 and receiver should be close together and have as a minimum a shared local earth plane.