Interesting that Clark is always placed in the foreground and Superman in the background on these covers, implying that Clark's is the decisive role. Superman is supposedly the dominant, aggressive side of the personality- yet Clark is the one who could walk out of the relationship at his choosing, implying that Superman needs Clark more than vice versa. Superman is generally smaller, or even a mere "image" hovering above like a ghost. This suggests he could not exist long without Clark.
Also the first cover shown was published very shortly after John Romita's classic "Spider-Man No More!" cover for AMAZING SPIDER-MAN #50. I recall that Carmine Infantino was designing all the DC cover sketches around this time, and trying to bring in a more dynamic, Marvel-style look to compete on the newsstands. Could the Romita cover have been an influence on the same basic design that kept being reused for Superman and Clark? (Although I'm not aware of any covers that show Superman and Clark actually walking away with backs turned to each other, as in the Spidey original.)
http://www.milehighcomics.com/cgi-bin/backissue.cgi?action=fullsize&issue=77906321824%2050