On occasion in the Battle of Britain RAF and German fighter pilots were on the same HF radio frequency and could talk to each other if they so desired.
Also on one day in the BoB all available RAF fighters were committed, and a WWI RAF fighter pilot, a Station Commander, was told there was a raid coming to attack his airfield and there was nothing to stop it. He jumped in his Mk1 Hurricane and headed for the German force. As it turned out, the German He-111 force, after getting thier arses shot off day after day, failed to rendezvous with its fighter escorts (unlike the RAF, the German fighters were never able to talk to their bombers) and were flying along fearfully, just knowing they were going to get clobbered big time. When the lone Hurricane attacked them, they said, "We knew it! The RAF fighters are here!", jettisoned their bombs and ran for home.
IIRC That station commander was Wing Commander Victor Beamish who was assigned to RAF North Weald on 7th June 1940, I'm certain I read it in and RAF pilot's autobiography a few years ago.