It is much simpler than you wrote. There is no need to send more coins to A2. Here is how it is done, assuming coins were sent from A1 to A2, but the amount of the transaction fee was too low.

1. Send coins from A2 to any other address (A3), and pay a transaction fee that is enough to pay for both the A1→A2 and A2→A3 transactions.

That's it. One step.