Got into an argument with a SCH/WHM player the other day. I said "I prefer rolling with 2 WHMs because they are smoother." He laughed at me, and proceeded to fail healing Garuda EX. Sorry, but I prefer 2 WHMs. You just can't beat 2x regen + 2x Med II for passive healing preparation. This is conditional to fights. Titan HM still works better with WHM/SCH. Can't speak on Titan EX because I can't get a group together who can manage spiney and sisters correctly.

Anyway, imho, 2x WHM > WHM/SCH and probably > 2x SCH.