Citroën build like 700, IIRC, BiMotor CV2's. I think they were called Safari.
I don't think synchronizing is much of a problem. If one engine is stronger than the other, the other one will just get a little push in the back. Just like driving downhill gives your engine a little push in the back.
The gears need to be chosen so that both engines max out at the same time. Otherwise you have to shift when one engine reaches its peak, in doing so not using the full potential of the other engine.
Bi-motor cars are interesting, and a way to make a balanced supercar out of a FWD chassis. But it'll never beat the single mid/rear engine.