Parallel motors on the same output
If you have a 12V supply and each motor has it's own rheostat, they won't affect each other. Cranking up the rheostat on one car will increase the total resistance in only that leg of the circuit, more resistance means less current and a slower motor. Now conservation of energy applies, but not as you've described above, the drop across the rheostat creates heat energy (a rheostat is really a glorified heating element), that energy will not be fed into the other car because the power supply is not trying to put out a fixed amount of energy, the power supply will supply only as much energy as is needed at a fixed voltage, the current is free to be whatever the cars need. When you have loads in parallel on a good power supply, they won't have any effect on each other.