If you looking for “historical correctness,” those opposing subs shouldn’t even have been dukin’ it out in the first place. In WWII you rarely (if ever) saw any sub vs sub battles due to their impossibility. The only way I think this would be feasible is if you waited for the other sub to be forced to surface (for oxygen) first before firing.
As far as sub rules go, I play with a couple, especially when it comes to detection (though these would require destroyers). However two I like to play with is sub-pens and wolf packs. For anyone interested:
Wolf-packs
If 3 or more Subs attack together, they obtain wolf-pack status and one sub (the leader) adds 1 to its attack value.
Sub-Pens
A sub may choose to remain inactive by using it’s non-combat phase to dock in a sub-pen (any coastal territory under the team’s control with an IC). That sub is considered “submerged,” until the beginning of that player’s next turn.