You can absolutely use just one. I do it all the time. I have had an 8-pack of coke bottles, a gallon of milk, and a U-lock and cable on one side with nothing on the other. I was nervous about it and thought that it would affect the handling, but I didn't even notice.

If I paid close attention, I could look down and see the bike slightly tilted, but I could not feel it at all.

If I were you, I would go the extra 20 bucks and get the Arkel utility basket. It has some outside pockets for keys, cell phone, and what-not and is a little bit bigger.