Its just your hormones at work. It is impossible 2 like 2 people
at once, unless you are undecided, or simply a flirt.
Don't be mistaken that just because she takes the initiatives in
starting MSN conversation, or that she reply long messages would
mean that she like u. Who knows, she may be just extremely
friendly, and she do that to other people too.
I would say, make up your mind, which girl u like, and then chase
that one. Do not try stepping two boats at a time, only 2 find
urself capsize later on. Even if u are currently not attached to
any of them, it is kinda wrong and stupid to try 2 chase both at
the same time, because the girl will feel that u are not really
liking her by chasing 2 at the same time, and that she may be a
back-up for the other.