Now there's obviously a lot of very good reasons (financial, logistical, etc) to not make a twin engined "pure" fighter. What I'm more interested here is purely technical issues, and what, if anything, could have been done to remedy them.
So, maybe?
Well, they violate the KISS principle.
Pretty much the only reason to resort to them is if you don't have a powerful enough single engine.
You are trying to use cleverness to overcome the drag of the normal twin tractor set up.
I am not sure were the two side by side engines geared together fall in here.
P-75.
It does show some of the problems with twin concept. You have to cool both engines and unless you are very very tricky that means twice the drag for the radiators and oil coolers.
Now for the US it also ran into the fact that by the time this thing flew in Nov 1943 (in cruder form) the P-47 was within a few weeks of being fitted with paddle blade props and water injection and drop tanks. R-2800 was giving about 2300hp compared to the 2600hp in the early V-3420 engine.
There was a tandem Soviet high speed bomber. But while skinny it was also freakishly long.
You get the small frontal area, You also have the crap view over the nose, and for a bomber, no good place to put the bombs.
In the picture one prop is turning, the other stationary.
By the time you fit a decent bomb-bay and enough fuel to actually go very far the drag has increase to the point where the tandem engine arrangement isn't getting you that much.
Now for a fighter you don't need a bomb bay and you need less fuel so maybe you fit that stuff in the wing. You do have a CG problem and there is no way to get around the poor view, unless you jack up the cockpit which kind of ruins the whole idea.
Kawasaki tried the cockpit between the engine trick like the VB 10 with the Ki-64
See
The Kawasaki Ki-64 was an experimental fighter that employed tandem engines and evaporative cooling. More pressing priorities during WWII resulted in the aircraft not being repaired after it was da…
If you are not already aware of it. Japanese tried to get around the cooling problem by using evaporative cooling in the wing surfaces.
also see
Technical Data Manufacturer: Kawasaki Kokuki Kogyo KK (Kawasaki Aircraft Engineering Co Ltd). Type: Single-seat fighter with tandem-mounted engines. Crew (1): Pilot in enclosed cockpit. Powerplant: One Kawasaki [Ha-72] 11 (Ha-201) twenty-four...
for some drawings showing the layout.
Let's remember that the drive shaft in the P-39 weighed about 50lbs and they needed to make the fuselage about 50lbs heavier to keep the bending to about 1in the 10 ft and they needed a universal joint in the drive shaft.
Actually getting the front and rear engines to play well together was often a problem.