There is a trick that has worked for me in the past on AF:
1. Passenger #1 selects window
2. Passenger #2 selects middle (as its forcing you to do)
3. Change Passenger #2 over to aisle

The coded algorithm will basically see Passenger #2 as a different passenger so it will let you pick the aisle seat that is next to it.

You may need to try this on both app and website. Let me know if that worked!