I have to refute your point #1.
There's no need to keep engines "in sync" when they each power their own wheel. the road does that.
The only thing to watch out for would be loss of traction on a particular wheel, but traction control would not significantly more complicated with two or four drivetrains instead of one. In fact, you could do away with the wheel speed sensors and rely on relative engine rpms to determine when one has lost traction.
The biggest reasons we don't have cars with multiple IC engines are:
2. Complexity (each would need its own transmission because as you mentioned, they work best at certain rpms)
3. Efficiency at high load (two 2.0l engines will eat more gas than one 4.0l)
4. It's unorthodox - you'd have to convince people its not stupid
Honestly, I think that there is a compelling argument for making cars with multiple engines, i.e. a transverse engine for the front two wheels, and another for the back.
1. you can re-use almost all of the parts from front to back, so while there are more parts, there aren't more UNIQUE parts.
2. Efficiency at low loads (ideally done with a stop-start kind of system, you could simply leave one engine off when the extra power and traction are not needed, thus doing away with almost half your frictional losses and reciprocating mass)
3. Redundancy - (if one engine is experiencing a failure of some kind, you have another one!)
4. Ideal AWD! perfect front/rear torque distribution when both are running. (All other methods of torque distribution have shortcomings, whether it's by viscous clutch, eletronic clutch, clutch packs, torsen, solid coupling, whatever. There is a trade off between slip and traction, and slip is needed for a car to turn without destroying gears.)