YES, you can use WDW passes at DL, we did it last year. The process was a little weird. I thought, go to guest relations right? NO. they want you to go right to the gate with your WDW pass. You explain that you have a WDW pass to the CM at the gate. They take it, and leave, and go somewhere and someone calls WDW to verify the pass, and WDW takes a day off the pass. Then the CM returns with your WDW pass and a 1 day pass to DL. (which you can use to park hop, and to get FP, etc).
It was ODD to me that you do not just go to guest relations for this. But maybe you could do that and then not enter the park after all, sell the DL one day hopper, I have no idea.
It is a GREAT way to save some $$, because my husband has done it several times using days off an old 10 day (no expire) WDW hopper. It is less per day than buying a 1 day hopper at DL. Some people only have one day to visit DL after all.
PS you cannot use DL passes at WDW.