It's interesting that Ray Collins, who was second only to Welles in the Mercury's radio branch, had almost no involvement at all in it's theatre branch. The only Mercury stage production he ever appeared in was NATIVE SON, which was pretty much concurrent with CITIZEN KANE. The same thing goes for Agnes Moorehead and Everett Sloane. Third behind Welles and Collins on the air, NS was also Sloan's only appearance on the Mercury stage. Moorhead only appeared in THE MERCURY WONDER SHOW.
Joseph Cotton, on the other hand, had important roles in most of the Mercury's stage productions, but appeared in only a few of the radio shows. If you peruse Jonathon Rosenbuam's chronology in THIS IS ORSON WELLES, you see that the stage Mercury and the radio Mercury were fairly separate entities...held together, of course, by Welles's unfathomable energy.