I am not sure about cars, but on electric heli's any time you use 2 esc's you need a mixer so that the two motors don't end up fighting each other on output. The EVX from what I understand splits an even signal to both motors while maintaining the same power to each. With ESC's (as with all electronics) no two are exactly alike in performance because of small irregularities in electrinc components resulting from manufacturing processes. This would result in a battle of the front end and rear end motors leading to bigger chances of an motor or ESC electronic failure.