Juggling two girls.

...Flip a coin?

Leave both of them and find someone new.

go with the person you can see spending the rest of your life with

Funny , but I you can't just turn close friends, would you ditch your closest friend for a rather attractive guy?

I don't have friends, so I wouldn't know.

Im sure you have friends, come on are in in sixth form or uni?

Doing my GCSEs..

Haha, you serious? You'd make an awful guy.

I'm so serious. I'm make an awesome guy! I do have a girl's mind, after all.

The reason I said it is purely because they're friends. If they were strangers to each other, then go ahead, drop one and get off with the other. Because they're friends, they'll often be seeing each other and the one you drop will be seeing you with the one you're with. Things will be unbelievably awkward for them. As a girl who knows girls, girl friends, and emotions, I'm saying drop them both.

On the other hand, you could find out what their relationship is like (if you don't already know) and if they're the type who are bitches to each other behind their backs, then go ahead and drop one and go with the other. If, however, they have a good relationship with each other and it looks like it'll be long-lasting, then drop them both and don't tell them why.

Are they both aware that you're with the other person?

"Are they both aware that you're with the other person?"
Yep and there is a growing animosity between them over it.

I simply don't want to ditch them both because then I'm left with no-one which is an unhappy state to be in, so no I'm not going to merely drop them both.

If you want to potentially break up a friendship (and cope with the drama that follows) then go ahead and drop one and go with the other. Clearly you've made up your mind. Even though I disagree with it, knowing how destructive many girls can be after a break up/friend loss, it's obviously your life and you choose to do what you choose to do.