Second, you don't know if you will be able to convert the TD on the second possession. Your likelihood of getting a TD and a FG increase if you go for the TD both times, rather than just going for it one time. You definitely need at least one TD; are you more likely to get it if you go for it twice than if you go for it once? Of course, if you find yourself with a 4th and 6 on the 25 on your first drive you kick the FG, but you've at least given yourself a chance to get the TD.