Depending on how late MK is open, we sometimes park at Epcot and monorail to MK and come back to Epcot on purpose (I find it easier to leave from Epcot than MK). You can also do a round-trip just to do the ride through Epcot for fun (DGS is a mega-train fan, so we spend a fair amount of time on the monorail). You can also use the resort boats. You never have to show a ticket to ride any of the transportation (so far).
MK is usually open later than Epcot, so you have to be mindful of that.
If you are changing parks late in the day, and leaving from the second park, you would probably want to drive to the second one.