Which one to do first depends on park hours. We hung at Epcot to watch the holiday version of IllumiNations then hightailed it over to DHS to get in before they closed at 10pm!!! Really wanted to see the Osborne lights. Made it in 18 mins...but we were not 'walking'..more of a race walk!! But we made it before they closed.
Normally, it is going to be better to park at DHS, check out the Osborne lights and then walk, or boat, over to Epcot. To find the walkway...exit DHS, stay to the left. Walk past the end of the canal (unless you ae taking the boat, in that case just walk down to the docks at this juncture). Stay on the sidewalk, parking lot to the right, canal to the left. You will get to a downward slope, with signage pointing you to BW. Take this and keep on trekking. It's a very pleasant walk...safe and well lit. I usually cut through the pool area of BW, then across the Village Green and on to Epcot. The walk takes about 20 or so mins, depending on pace. After you watch IllumiNations, you can either boat back to DHS or just walk back. The boat ride is 25 mins...but you do have to add in wait times...and at park closing, the waits can be lengthy.