Motorcycle tank question..please help!?

Ok on my Harley 1200 sporster it has a aftermarket tank that I didn't put on just bought the bike and the tank has two caps and when i open up the right tank its empty and when i open up the left cap its full. my question is is there something i hae to switch over to use the other side of the tank or will it automatically do itself? Or is mabe one side reserve? I have no clue and I dont want to run out of gas on teh way to the store.