For what it's worth, I've stayed at both Pop Century and AoA this year (over 30 days total within this year alone, I know I'm crazy). I'm from the West coast - due to time zone differences I go to the parks late and leave after the park closes. I've never seen them share buses, with the exception of one day during a hurricane and the park closed at 2pm. There were women sitting on a curb shaking after being drenched by rain, they sat between the bus lines for PC and AoA after feeling defeated by an overflowing Pop Century line. The bus driver noticed them, walked over, and offered to drive them to Pop Century. He dropped everyone off at AoA before taking them over to their resort.
The bus driver himself said that it was abnormal as each resort had their own bus line. He was at the end of his shift and it was his last run, as someone else was taking over for him and was on their way from the bus lot.