Only reason I can think of for the paired rear exhausts is heat & flame too close to the airframe.
They do spit fire at times.
Didn't that aircraft have a lot of wood in its construction ?
(Something like that was called "the flying piano" for its wood content.)

Love the sound of a Merlin or Griffon flyover at speed. Nothing else like it.
Wonder what they'd sound like with an exhaust more like the old V12 Ferraris...?

As for the RPM, I would think the props are part of the equation.
And back in those days engines weren't apparently spun at high RPM in most applications.