Yes, there are Park to Park busses - they should start running mid-morning, like 10:00 AM. Check the signage for the bus stop numbers, but they will usually on the further-away stops.
Do note that for parks connected by the Monorail or Skyliner, there may not be busses. MK to Epcot can be done via Monorail, and the Skyliner can take you between HS and the rear entrance of Epcot.