Pojo,

This happened to me recently at a Tournament during sudden death which cost
me 1st Place.

I had Chansey active with 2 DCE's versus my Opponents SandSlash.  I have no
bench and my opponent has Ditto on his bench.  I Double-Edge SandSlash and
this knocks us both out.  I did this to reset the game to another Sudden
Death since I did not like the current set-up. 

It was ruled since my Opponent had a bench Pokemon and I did not, I lose. 

Is this correct?

Thanks - John

Yes.  He won with 2 conditions, you with 1.  Thus, he wins the game.

Scott