Tiffany did this for me in December at no extra charge. I was dropped off at Dolphin and picked up at Boardwalk. However, those resorts are very close. I'm not sure if there would have been an extra charge if they'd had to pick me up from a totally different area.
I have seen others post that one of the other towncar services (Happy Limo, I think) charges for two one-way trips in this scenario, which of course would drive up the cost.