The morning park buses do not share between the resorts, except when they send an extra "pickup bus" to get people who didn't make it on the first bus. Every morning bus that we took to MK or AK in the morning only went to one resort, except for the one time we were on the pickup bus, which had gone to BC first, then to YC. We took buses from both YC and BC because we often went to BC for breakfast at the marketplace. The afternoon buses went to both resorts.
The return buses, from park to resort, went to both YC and BC.
We were there in October, January and March and the bus situation stayed the same for us for all three trips.