The compound probability of scoring 1 PAT followed by a 2 PAT is the same as the probability of scoring a 2 PAT followed by a 1 PAT, as long as the assumptions for converting the 1 PAT and 2 PAT remain the same for both the first and second touchdowns. Thus the expectation of getting a total of three extra points after both TDs are scored does not matter which order they are obtained, i.e. Prob(1 followed by 2) = Prob (2 followed by 1).
However, when looked at individually, it's easier to complete the 1 PAT than the 2 PAT. So if you have decided you will play for a tie in regulation, after scoring the first TD (6 points), being 9 points down, the less risky play initially is to kick the 1 PAT, knowing that you will need to score 8 more points, which is possible if you obtain one more possession. Although one more possession is remote and the probability of scoring another touchdown is even more remote, you are technically still alive with the very limited time remaining in regulation.
In a sense, you can look at this as what is the probability the next play (onside kick) actually matters? In both cases, whether 2 followed by 1 or 1 followed by 2, it MUST be recovered but if you went for 2 initially and failed, converting this onside kick is not sufficient by itself to keep you alive, you must gain another possession. So, in my way of thinking, you take the more sure thing and kick the PAT, keeping the one possession game in play.
The key determinant in this scenario is the time remaining in regulation. Given more time, you might initially decide to go for 2, but in this game the chance of more than one possession was nil.