Moral complaints aside... (which i do have... but i don't bug my friends who cheat so i won't bug some random internet guy about it.)
I would say you are already screwed. The fact that neither knows about the other is deadly.
Though you do have a chance if you break it off with one immediately and shrink the time line.
The world is a smaller place then you'd think... especially with the internet. You'd be surprised who people will meet and become friends with in the future.
All it takes is one to become friend of a friend of a friend with the other... many many years down the line even after you'd broken up with the other for you to be screwed.
You could be married to the first girl, broken up with the second for years... then she could become friends with the sister of one of your wife's friends, friends... and your screwed. She hears about you and says something off hand like "Oh I dated him in 08" or mention a concert or anything that establishes a timeline...
and suddenly your faced with a divorce. I've seen in happen. It ain't pretty.
You only have two choices if you want to make it out with either of em.
A) Pull out now. Get rid of one right away, so that whenever the landmine drops... and it probably will you can screw with the timeline and say something like "it was before i met you." or "It was when we broke up for that short period of time"
The longer it goes on... you can't screw with the timeline. Your fucked.
B) Break up with the girl and Come clean now. You somewhat defuse the situation. Claim it was a one time thing and you screwed up... and if it ever comes back later, just say she was lieing for saying it lasted longer.
