Most plans I've seen put more training mileage on Saturday and less on Sunday, say in a 70/50 ratio. I don't know why that is, but I'm guessing that during the actual event you will be hyped up to finish the second day's ride, so you don't have to train for it as much. And during training, you'll want to go from the Sunday ride to another training ride on Tuesday, so you can't be too wasted. So maybe that's it.
I've never done back to back 100's. I thought it easier just to do the whole thing all at once and have done with it.