Well, it isn't rendering to each, just rendering once and copying the results to each :) I believe you can disable this copying of the image in the Preferences (the overall Preferences, not the Render Settings dialogs, you can use Corona menu -> Preferences to open this, or just C4D preferences, same thing). Since it's just a copying of an image, it is unlikely to have any noticeable impact on things (and the advantages of having the two in sync probably outweighs any small difference there may be).
