How can having two girlfriends at once go wrong, if they both know each other and know about each other and everything is in the open?

Will there be jealousy after time? Are there any statistics on how this kind of thing works in most cases, I’m guessing it can’t end well really.

They have both known each other since school and are almost 30 now, and they are still good friends after all that time.

What can I expect if I keep seeing them both at the same time, tonight seemed to go ok, but can it really last?