my thoughts on what you are suggesting
1) load capability of the differential. Can it handle the torque/power from 2 motors?
2) traction capability of the tyres. Can it handle the torque from 2 motors, or will it just start spinning from the additional torque. (at higher speeds I guess it may be better, but from standstill this issue will be there)
3) is the belt connecting both motors able to withstand the torque of the motor and not just snap?
This concept is similar to what Koenigsegg is doing with the Regera/Gemera rear drivetrain. however for you, instead of using an ICE engine, you're using the Model 3's motor.
This is slightly more expensive on the motor side, because hub motors are fairly expensive for the power they provide. a cheap one used on bicycles will only give you single digit kW. you can go a bit higher to get those used on scooters/bikes, but the size (thickness) gets bigger and may affect the car's handling performance with the shorter CV axles.
The money-no-issue option is to use the same pancake motors as Koenigsegg (Yasa, or other brands) which are super thin and provide lots of power (>200kW each).
The basics on why this is not a good idea was already covered by Duncan. I would add that the addition of the complexity of introducing the wheel and mechanisms to hold it makes this idea not very good for your purpose of increasing performance
This may help improve performance in theory. However, you need to watch out for a few things:
1) load capability of the differential. Can it handle the torque/power from 2 motors?
2) traction capability of the tyres. Can it handle the torque from 2 motors, or will it just start spinning from the additional torque. (at higher speeds I guess it may be better, but from standstill this issue will be there)
3) is the belt connecting both motors able to withstand the torque of the motor and not just snap?
Among your 3 ideas, my opinion is this might be the best one (least amount of work required with decent performance gain, but slightly complex on the electrical side due to different voltage of the model 3 motor and hub motor). Still agree with Duncan that other options like lightweight-ing the car is better.
This concept is similar to what Koenigsegg is doing with the Regera/Gemera rear drivetrain. however for you, instead of using an ICE engine, you're using the Model 3's motor.
This is slightly more expensive on the motor side, because hub motors are fairly expensive for the power they provide. a cheap one used on bicycles will only give you single digit kW. you can go a bit higher to get those used on scooters/bikes, but the size (thickness) gets bigger and may affect the car's handling performance with the shorter CV axles.
The money-no-issue option is to use the same pancake motors as Koenigsegg (Yasa, or other brands) which are super thin and provide lots of power (>200kW each).
