I think it’s definitely possible, but maybe over complicated without achieving much.
I suppose it all depends what your main objective is. I think in this case, to improve efficiency & also have the big torque you want, you should just use a big single motor such as a 6374 size motor. These big motors have lots of torque and can climb hills easily if they are fed enough power which you are controlling as the rider. They are also using less power when on the flats. Here is one for reference.
What you are describing, regarding sensing increased load when starting to climb a hill, this sort of happens anyway. This is actually the job of the most motor controllers, as the load increases the motor controller will send more current until the load decreases or something burns.
In the case of the VESC you are actually controlling current (if you have that option selected) So as you reach the hill, to maintain your current speed you must increase the current to the motor which you will do by pushing the throttle.
If you have the VESC in Duty cycle mode the VESC will automatically increase current to maintain speed until it reaches its maximum set current or the load decreases.
Some of the new eboard remote controls on the market at the moment have cruise control, Once activated it will try to maintain your speed without the throttle being used. Soon I will test this feature using Current Control & Also Duty Cycle and see what has the most desirable results.
Anyway… I think a single large motor will probably always be a better option for efficiency than two smaller motors. It is also cheaper & simpler to build.