Eh...
There are two fundamental questions you have to ask:
1. Are you sure the second girl actually likes you? They both may act the same around you, but that's no solid indication. Every girl has a different baseline of behavior, and deviations from that are indications of emotional attachment. If the first girl is normally outgoing, but acts shy around you, normally doesn't laugh at jokes, but does at yours, and goes out of her way to be around you in a way that isn't normal for her, she likes you. The same metric applies to the second girl. But, if she's normally shy (no moreso around you), normally very laugh-happy, and says hi to all of her friends, it's less a sure thing.
The point, fundamentally, is not to jump to conclusions. If you go for the second girl, and she doesn't like you, you may lose your shot with the first girl. Chickens, hatching, all of that.
The second question is simply: who do you want to go out with? If you're dating either of them out of a sense of needing to have a girlfriend, you're doing no one any favors. No harm can come from pursuing neither of them until you actually know what you want.