For the math.
If you have a 40% chance to make the 2.
You go for two because if you have a 40% chance to have made it and thus scoring the second TD gives you a ~98% chance to kick an XP and go up by 1/win. So call that a 39% chance to win the game.
If you fail the 2 point then you have a ~40% chance to go for two and level it and 50% chance to win in OT. so ~20% additional win chance. (Edit: Sorry not an additional 20%. It's 20% * .6 since that only occurs 60% of the time so 12%)
If you fail both you lose.
So that's ~52-3% win chance by going for 2 versus 50% win chance (or 49% if you count missing an XP in there and thus losing anyway) if you go for 1 both times if you think you have a 40% chance to make an 2 point conversion.
Obviously this is all dependent on scoring two TDs, but if you don't do that then your choice really doesn't matter anyway.