Both couples must have thought that the other couple would help them up the rungs of the celebutard ladder and that didn't work out.
Or Kelly and Mark could have had some initial superficial things in common with Howard and Beth to start a friendship only to discover Beth is a woman child and Howard is a douche.
Or maybe they're still friends - Howard is the master of secrets. I remember when Howard announced that Mark was marrying them and thinking, "huh? who? why? where did this come from?" Now that I think about it, that choice of officiation has to be the cement drying on Howard's transformation as a PC Libtard.