I was thinking that the electric engine would be used to drive the vehicle as far as the charge would take it, then you would start up the regular engine and drive the rest of the way.
( So in that particular case you actually would just need one gas pedal / acceleration pedal.)
You would have the non driving engine & transmission in neutral while the other motor was used.
Another option would be to have the engine act as the generator to charge the batteries . It would be less cumbersome than a tow along generator.
You could drive along for 10 miles or so, turn on your 'generator' for a few miles and then turn it back off again once the batteries were charged enough to get you ten more miles ( what a pain in the a$$ that would be
)
Even if you left the 'generator' engine running, you would get really great gas mileage, since it would just be at idle the whole time.