The buses don't pick up at one theme park and then take you to another. You'll either need to transfer to another bus via bus, monorail or boat.
For example if you want to go to AK from MK the best way would be to take a boat or monorail to one of the MK resorts and then pick up the bus to AK from the resort.
The monorail will take you to the TTC from MK, then you have to get on a different monorail to get to EPCOT at the TTC...of course this works in the opposite direction (going from EPCOT to MK), but you still have to transfer monorails. (no bus needed here though)
When parks start to close you'll need to transfer buses in DTD.