P38 Q & A?
A few more items to consider when flying twins.
The phenomenon that rottilover hinted at is called Minimum Controllable Airspeed. On a twin this airspeed may or may not be the same as the aircrafts's stalling speed. By definition it is the lowest airspeed that the aircraft can maintain straight flight with the critical engine inoperative. His experiencing flat spins in the computer P-38 is caused by this effect. The single engine with its displace thrust line is producing too much yaw for the fin(s) and rudder(s) to correct. So the simple solution, as he has indicated is to either throttle back so that the engine is pulling less or fly and land hotter to keep the rudder(s) effective.
And
put your weakest engine on the right wing.
And
Get both engines to idle and accelerate reliably.
And
Set peak power on each engine as if the airplane were a single engine model. If you try to exactly match high end on both engines one or the other is going to be running rich or lean setting the foundation for an engine quitting. The small differential thrust you are going to get at the high end because of slightly different rpms will not be a factor in the air.