The Vega Starliner had a coupled powerplant - two Menasco air-cooled inline 6 cylinders driving
a single prop.
The MC72 used the FIAT AS-6.
The Fairey Prince and Monarch were also true twinned engines.
The Napier-Halford Rapier, Dagger and Sabre were basically two flat engines mounted to a common gearcase, all three types had a separate crankshaft for each 'engine'. Ditto the R-R Eagle.
As to improving the cooling of the aft cylinders.
After problems with the original installation the cooling air for the Armstrong-Siddeley Deerhound radial was taken in at the front by an under engine scoop and was then conducted across the cylinders from back to front. The engine was test flown on an A S Whitley.
The D-H Gipsy 12/Gipsy King inverted V-12 air-cooled engines of the DH.92 Albatross were cooled by air
taken in at the leading edge of he wing that was then ducted forward over the cylinders to exit out the bottom of the cowling behind the propeller.