First, you shouldn't have said you didn't care when you actually did care. You might have saved yourself a lot of grief by telling the truth.
I know it's awkward for a lot of reasons. But there's no going back now, so you're just going to have to deal with it. Start off on the right foot and behave politely and with maturity towards both of them.
I think you can help yourself out a lot by putting things in perspective. Remind yourself that you had only two dates with this girl, and things fizzled out. It wasn't a long term relationship. The two of you were not committed.
In time, the awkwardness will fade, and you will look like a star for not getting tripped up by this situation.