We've killed quite a few combos of batmen in the nexus, but not a single fight has counted towards the combos for Finite Possibilites
I submitted a ticket also about this but the GM said that no one has reported the problem so they could not verify it. Here's the response I got from a GM: "Unfortunately, this feat cannot be verified. I can tell you that there has not been any other reports of this but we will definitely keep an eye on it. At this time I'd like to ask that you try again." So we should file as many tickets as possible about this so that the devs take notice.