I don't pretend to know anything about this stuff, but this issue of having to balance the two engines together makes me think of twin-engined kitcars, which are all the rage at the moment. These typically use two engines from high-performance motorbikes such as Honda Blackbirds or Yamaha R1s, driving through a custom transmission that links the engines together - but the bit which is relevant to this discussion is the fact that both engines still each retain their own gearboxes, specifically in order to allow the two engines to rev independently.
From what I've read about this kind of set-up, the driver has the choice of either operating both gearboxes together through a linked gearlever, or he can change gear in just one gearbox or the other - which would obviously mean that the engines would be operating completely out of sync with each other, and yet while still being connected together. As I understand it, being able to mix-and-match the power and torque output of the two engines like this gives better acceleration and performance than if both engines are performing identically (but presumably requires a very skillful driver).
I'm not suggesting that this is remotely relevant to buses (especially as the rpm of a sportsbike engine is 10 times that of a heavy diesel), but just thought I would mention it in passing