Note that Airs through the 2011 model came with 2 - USB 2.0 ports; starting with the 2012 model they now ship with 2 - USB 3.0 ports.
USB USB 2.0 powered HDs are designed to work within standard USB 2.0 specs, i.e. max 0.5 A @ 5V. If you are getting 2 external drives to work through one hub that is pretty good already, as together they are probably drawing greater than the USB 2.0 spec.
The Air's USB ports are purposely over-designed to power an external Superdrive, which pulls 6-7 W (a bit over 1 A @ 5V, which is actually greater than the USB 3 spec of 0.9 A @ 5V). That's what makes an Air great at charging Iphones, which similarly pull about 1A. I would recommend running "overpowered" devices like the Superdrive or iPhone directly from the Air's USB port, and not through a USB 2.0 hub. If you need to run a bunch of lower power USB devices, too, it would be fine to run those through a hub on the Air's other USB port. Mainly my $0.02, but I am a very happy owner of a 2011 Air which has worked very hard over the last 14 months, and shows no signs at slowing down.
I plugged it into my 2011 Mac Mini and it charged my iPad, so I think there is a good chance it will charge yours. Remember the iPad should be charged off of 2amp, but I have plugged it into a 1amp charger and it has charged even while saying that it isn't charging.