Is it useful for twin propeller airplanes to have twin vertical stabilizers, in line with the propellers?
The vertical tailplane is dimensioned as a tail volume: total vertical area times the moment arm to the CoG, to tackle the following issues:


Control after engine failure.
Lateral Stability.
Crosswind landings.
Spin recovery.


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.