Just as you suspect there are a lot of factors affecting any conventional multi with an engine out. And you are correct in the assumption that twin fin/rudders can provide more yaw authority. Of course the reason is with a twin with an engine out 50% of your rudder surface is operating in high energy air behind the good engine operating at high power although you have lost some total yaw authority with one rudder operating in lower energy air behind the dead engine. The net effect is greater control than a single tail aircraft where the rudder is operating in the lower energy air at all times.

This of course is the reason twin fin aircraft were developed many years ago and used by countless types. To answer your question about the specific RC types you mentioned you will find the answer in tail volume. Tail volume is the sum of the tail area times the tail moment. The greater this figure is in the case of the vertical surfaces the more rudder authority and yaw stability will be. This equation can be changed by adding/reduceing the surface area or the moment arm of the tail.

If you look at both the dualist and the Hornets tail moment even though the vertical surfaces may not be huge you will see both ships have very long tail moment arms when compared to both the B-25 and P-38 and the 38's vertical surfaces as well are smallish.

John