Using external USB hard drives via a USB hub connected to a MacBook Air
There is no 100% rule that applies in all situations. 

For example:


there are portable USB hard drives that do not require their own power source
there are portable USB hard drives that do not require their own power source with some computers, but do with others
there are USB hard drives that do require their own power source no matter the computer
some USB hubs are powered hubs, while others are not
some USB ports deliver more power than others


My recommendation 

If you wish to connect external hard drives via a USB hub, especially is you're going to connect two of them simultaneously, then you should invest in a powered USB hub. By a powered USB hub I mean one that has its own power source. This way you're not relying on the MacBook Air to power the USB hub and two external hard drives simultaneously.