If Guy A and Guy B are both friends they will both find out eventually. I am at a slight loss though as to what advice you want, in fact I am also confused as to which guy you fancy A or B? I assume that you mean Guy A has left and B who you fancy is left on the scene.
My advice therefore is to stop fooling around with all the guys of the alphabet. I am being facetious of course.
I think if you really like Mr B, then why not ask him out. But you might as well be honest from the outset as he will find out about Mr A eventually if they are still in contact. As an aside, enjoy yourself, you are only young. But I wouldn�t be too willing to fool around too often.