Protect device from losing power when to much current is drawn
This is not what you want to hear, I think, but the way to do it is to provide 2 power supplies - one for the pi, one for the motors. The pi supply will not need to provide much current, so it can be smaller and cheaper than the motor supply.

As a bonus, should you decide to upgrade to more power motors which require a different voltage, you're already set for the pi.