Well, interesting to say the least.
Personally I don't think it really matters whether an ace shot down more 2-seaters than single-seaters. Who can say that it is less hard to shoot down 27 2-seaters than it is to shoot down 21 single-seaters (Voss)? The Germans never had the numbers to go out on enemy turf on equal terms. They were capable of meeting the enemy on their own ground which in the end happenened because of the Allied strategy. Why do people insist on measuring a person's bravery by the percentage of fighters shot down? What does it prove? I can understand that people look at these percentages (it proves what one wants to believe) but do you really think Bishop is a better airwarrior than say Voss or Richthofen because he's supposed to have shot down a higher percentage of fighters? Hartmann during WWII shot down 352 planes, about 90 of which were twin-engined the rest single-engined fighters/attack planes. Yet a lot of people rate Marseille and Nowotny as better airfighters.
Aces shot down aircraft, it was their job. You cannot expext Richthofen to say upon confronting a lone 2-seater:"Well too bad, it is a 2-seater! I have to shoot down a fighter today or else history might remember me as a coward or a lesser airfighter then my Allied adversaries." Richthofen does not decide what aircraft he will meet in the air.
In recent years I have only too often found that people try to downgrade the performance of pilot X in favour of pilot Y. Usually because of personal motives. Remember the controversy of
Ira Jones'statements on Mannock's score? Or the people who actually believed Richthofen had his kills set up for him? The German WWII aces suffered a similar, even worse fate, being accused of having propagande scores! MvR may have shot down a lot of 2-seaters but he also complained regularly about the lack of personal dogfights due to the changing of the air war. He wanted to shoot down Allied planes in personal combat, but the needs of the German war commitment demanded otherwise. For the Germans it was of the utmost importance to shoot down info-gatherers. MvR just happened to be bloody good at it.
Besides, I think the victory-systems of the Allies leaves a lot of things to be desired. You can first prove to me that Bishop and Collishaw shot down as many as they are accredited. Germans were meticulous administrators and this also provided an excellent victory-system. It wasn't foolproof, but it was probably as close as it could get in those days. Same goes for their system in WWII. Of course it was easier to confirm kills made over your territory, but they (on the whole) did not claim kills or OOC's DD's etc. if they didn't have a wreckage to show for it.