I'm just the opposite to bse, I only use the 5 day program anymore. Yes it is one more time through the chute, and one extra shot of pgf2a, but I've had better luck getting everybody to show heat at the same time. Last fall for example we synced up 10 cows using the 5 day, and timed Aied all of them. We didn't give gnrh to the 6 that showed a good standing heat, but we did to the other 4 at breeding that we didn't see standing. I realize that it is a small sample size, but we had 9 out of ten stick. We also had one other cow that just happened to line up perfectly on her own with everyone else, and she also stuck to ai. All told we had 10 out of 11. A normal percentage with the timed ai is usually 55-65, so my results were atypical.
The reason you give the second pgf2a shot at 8-10 hours is that there has to be a corpus luteum present for the pg to work on. Pgf2a regresses the CL and initiates the onset of estrus. Some animals might not have a CL at the time of the first shot, but will 8-10 hrs later. Hope this helps.