As was noted, USB 3.0 power is only supposed to go one way - from the USB-A connection to the destination. There's really no way to charge it with that port. If you attempted to use a USB-A to USB-C cable (or via various cable/adapter combos) what might happen is that your MBA would probably charge the power bank.
Now a newer MBA with USB-C/Thunderbolt 3 can charge off of almost anything (USB-A power adapter, power bank, USB-C, or even off another computer) although it adjusts to the available current. I'm not quite sure what would happen if two were connected to each other though.