Welcome to A2K. Certainly, feel free to ask questions as you see fit. While many of us are not relationship experts, many of us have had lots of life experience from which you might benefit.
Believe it or not, your parents might also be able to help answer some of these questions, too. After all they've known you all your life. However, I recognize that sometimes it's just too hard to ask parents this stuff.
#1 is a fantasy girl. Deal with reality by asking her out if you can keep your mind from racing ahead of yourself. I'm going to take a wild guess and say that you are a teenager or under age 20? Allow yourself a healthy fantasy, but do you really want to be boyfriend #4 or #5?
#2 is already interested in you and vice versa. You've a real rapport with her.
So ask her out and see where it goes. If you find you still fantasize about girl #1, ask her out, too
Just why is it you think the asking both girls out might be mutually exclusive?