What maximum voltage will I need my ESC to be rated at?
Connecting in series is a bad idea. Your may lose control and crash. Also, one ESC cannot drive two brushless motors, either series or parallel (if they are brushless). Even with a 56V ESC, a 56V battery will still burn out a 48V motor, let alone what you want to do. Numerous problems with your approach. You need to go back to the drawing board. Also, the ESC needs to be rated for the max voltage in the circuit. the motor voltage rating is just a number. It's not a voltage that appears in the circuit. However, the battery voltage IS a voltage that will appear in the circuit though.

There is no "remainder" voltage. Your understanding of what the ESC does is deeply flawed. An ESC is not a resistor or linear regulator. It's a valve that passes or blocks ALL the voltage of the battery. No opening halfway so if it blocks, it must withstand the full voltage of the battery.