Maybe one girl is a lot of fun, but you can't see yourself building a relationship with her. If you're seeing these two girls every day, then your decision is urgent. Ask yourself whether you seek something serious or something casual. Stick with the decision you make, and don't waste time second-guessing yourself. Tell her exactly what you want from her, and tell her that she's the only one you want provided that you are, indeed, prepared to commit to her and only her for the time being. Be simple, honest, and pure.