I tried 1-armed OHS after seeing a Steve Maxwell dvd (yeah, I tried tehm with a kettlebell too). I definitely noticed more ab/oblique/serratus work. Then again, I had a slight twist/lean away. I think the 1-arm lends itself more to a bit of a lean than the 2-arm version.
I find that it's more difficult to keep both DBs "directly" overhead, compared to a barbell.