Was that you cannot "hop" between DL and DCA using a WDW park hopper pass. Instead, what you do is go to Guest Relations (ticket sales CMs won't be able to help) and what they will do is give you a one day-one park pass to DL or DCA for each day you have left on your WDW park hopper pass (plus options on WDW passes get you nothing at DL/DCA).
We were concerned because our 4/5-4/19 trip to WDW was going to be our last ever (well, we expect it to be but 6 years from now, who knows) and we had 7 day plus passes. The way our schedule looked we were actually going to have a day left, that's when we found out we could use that day at ONE park in Anaheim.
Needless to say, once in Orlando things changed and we ended up using all 7 days at WDW and have nothing left to use in Anaheim (well, there is good ol' fashioned cash). But it was comforting to know in advance that leftover days wouldn't be wasted.