In some twin propeller airplanes, there are two vertical stabilizers which are on the same line as the propellers.
Is this shape helpful in terms of efficiency?
Like this one (twin boom tail):
The vertical tailplane is dimensioned as a tail volume: total vertical area times the moment arm to the CoG, to tackle the following issues:
The vertical tail is subject to side forces, and the aircraft structure must be able to withstand the resulting reaction forces. The fuselage is very suitable for absorbing these by applying counter-torque, and is the go-to place for mounting the fin.
A large, long fin has a relatively long moment arm, and it can be beneficial to divide the vertical tail into multiple surfaces, provided there are suitable mounting spots. And behold, the P-38 had two tail booms and a horizontal connection bar, and the natural mounting method is two fins each at the end of a boom.
The fins are indeed in the prop wash and this does help their efficiency - in straight flight. At a sideslip, large angle of attack or in a spin, the prop wash will be directed away from the vertical tails, and these will be dimensioned as if there was no prop wash at all.
Yes it is, but many designs are more stable with one large V stab in the middle.
Some fine Navy engineers took the B-24 Liberator and turned it it the Privateer, making long flights over the ocean easier on the pilots due to improved stability. The large V stab also plays an important role in avoiding "Dutch rolling tendencies", and, with its accompanying larger rudder, controlling side slip.
Putting control surfaces in the prop blast is an early form of directional control from thrust "fluid" flow, also seen in the V-2 rocket. Smaller fins in stronger flow, it works.
So a designer could consider all three, as in the Constellation, remembering Rule 1:
FLY IT AS A GLIDER FIRST!