You could park at DHS and then boat over to Epcot first thing in the morning. Then, you can walk back to DHS when done at BW.
In all reality though, your best bet is to choose a day that Epcot is open late enough to allow you to walk through. OR...just pay to valet park at BW. Or, you can probably park behind the Hess St., across the street from BW's entrance.