Efficiency / efficacy comparison between two hub motors and one central motor

Those are great questions and I'd love to know the answers, too. A couple thoughts ...

There are at least two companies making large size hub motors. PML FlightLink has long had some promise of supplying full size vehicle hub motors. And there's at least one company in China doing so as well. The most powerful motorcycle hub motor I've heard of is 7kw which is minimal for a car.

Most electric cars don't care much about gearing. GM's EV1 proudly did not have a transmission and went nearly 100 miles/hr.

For the differential effect .. this may be one big reason to use the existing drive train. You'd be reusing all the engineering the manufacturer put into a properly designed differential. Otherwise you'd have to invest some effort in designing the controller set up to properly manipulate power to each wheel to mimic the differential effect.

Here's some answers

2) Motorcycle hub motors probably aren't the best choice for you. The current hub motors are in the 5kW range. Two hub motors would be 10kW which is probably a little underpowered for what you need. Another way of looking at this is to compare the final weight of the vehicle. A scooter is in the 350lb to 500lb weight - a sub compact will be much more than twice that. They also don't have the same mounting arrangement (most are designed to be mounted with an axel that is supported on both sides).

3) I think the gearing would represent a big issue for you if you use motorcycle hub wheels.

4) AFAIK there are no true hub-motors that support gearing (in this context a true hub motor is one where the windings are inside the hub with the magnets outside of the windings). Note that Vectrix uses a coaxially mounted traditional motor and they use a single ratio planetary gearbox mounted in the hub. So it's kind of possible (but it's not really a hub motor at this point and will take a fair amount of engineering for a self-conversion of a compact)

5) If you want independent wheel speed control then you will need a controller per wheel. I'm not sure of the most effective way of controlling a multi-motor setup. But I'd love to find out! One day when I have time (ha! ha!) I want to do a 4-wheel drive, 4 motor, 4 door pickup with a range extender in the bed. Perhaps I'll call it the 4x4x4 :-)

Bottom line is that a multi-hub-motor design is tempting and would be a great project but (a) you'll need to source hub motors designed for cars not bikes and (b) it's a lot more complex project than going with one motor.

E-Traction has a double motor in one housing with a shaft out each end effectively making an electric differential. I've been kicking around some ideas for an electric PU and I an pretty certain I will not be using the existing drivetrain. There is too much inefficiency and loss in all the gears/bearings.

There may be some reasons why one might want to separately control the torque to each individual wheel (such as traction control, etc.), but you shouldn't need to do this for the purpose of simulating a differential. The main purpose for a differential in a vehicle with a central motor is to supply the two wheels with identical or almost identical torque while allowing them to rotate at different speeds. With hub motors, the wheels are naturally independent (there is no physical axle connecting them), so if the two motors are being driven to the same torque, this should happen automatically.