Servos twitch because the supply voltage isn't stable enough under load. Two on the same lead is not a problem provided that the lead can pass the current needed without changing the voltage. Servo positioning relies on comparing one voltage with another, one is got from the supply, the other from the control signal. If the load of two motors causes the supply voltage to shift even a fraction, that will be the start of the servo going to a new position. When it gets there, the load changes and away it goes again. If the supply into the receiver is borderline inadequate, the bit of the receiver that creates the timing pulses could also be affected.
A work-around is to add a 0.47μF capacitor across the red and black of each servo lead to slow down the voltage changes.
