Indeed, it is a simple trick, but maybe a bit too cheap.. It wouldn't benefit other use cases than that particular one it seems (unless someone has some other examples where such a switch could be necessary?)
A solution to that would be the Particle events and actions, which have been on the drawing board for a month now, and that we'd like to start to implement for the next release (ideally !!). Those events are a bit of a refactoring for the particles so we'll see how long it'll take, but this would allow you to simulate different particle systems within just one, not needing a second or third "Emit particles" modifier to do that.
The "Emit particles" in that case would only be used to emit particles from a mesh (fbx, box, plane...). One particle system would satisfy the *nested * scenario, and your problem with the simulation speed not being shared would vanish !
This one seems like the one true solution