I've never even considered this approach before but I think it would have been a stroke of genius to do this and one that A&M's marketing team should have gone with once the duo became established (from 1971 onwards). It would have clearly delineated their roles within the duo, with she as the group's "front" and he as the one running the show from behind the scenes. Of course, I doubt either of them would have been happy with that for one common reason: both would have seen it as relegating and diminishing Richard's role. But as a marketing tool it would have been brilliant. There are other musical collaborations between artist/producer duos of that era which didn't feature both artists on the cover. Bette Midler/Barry Manilow and Dionne Warwick/Burt Bacharach immediately spring to mind.