Hi
I don't know if this work for the allied side too,but i think it should work.
This is what happened to me.
I had one destroyer and 2 u-boot in the England zone,like the image below.
When an allied convoy enter the same zone,only the destroyer was ready to enter the fight.
Then i click over the England zone and after that the 2 u-boot was in list of ship ready to engagement.
Both kind of ships are now into the battle.
I do not see any reason because this way shouldn't work for allies too.