D.W.
Since you only arranged for 1 girl to baby sit, I would pay her a reasonable rate and hope that she offers some of that to her friend (who will, no doubt, be helping in some ways).
I'd also recommend providing pizza or something so they're fed as part of the incentive for helping out so late.
I, too, used to babysit alone that late, but I was only a few doors up from my parents' house and was watching kids I knew really well.
That's what I'd do personally - good luck making a decision. I hope it goes well.