can't connect two hard-drives to my airport
If the USB hub is bus powered (it doesn't have to plug in to a wall socket) then the problem is that your Hard Drives are drawing too much current. USB Standards state that each USB port needs to be able to output 500mA. 

When you divide that by four (assuming your USB hub is 4-port), you get 125mA, which isn't nearly enough to power two hard drives.

I suggest you get a externally powered USB hub (one that requires a power supply other than USB), as that will ensure that each USB port receives 500mA each.