When you have your first picture taken, that picture will sit there for 30 days. You have those 30 days to claim it. Once you have claimed the pictures, you have 30 days to do something with it, and after that it expires and leaves your account.
So if your timing is, say, first day at parks Sept 15, a picture taken that day will sit there until 30 days later (since Sept has 30 days that should be Oct 15, right?), when it has to be claimed. So, say to be safe you claim it (by entering the photopass card ID number on your account) Oct 14, you now have 30 days after that to do anything with it.
You can indeed put more trips than one on your account, and they could all be included on the photopass CD (which is, what I assume, you're going for) if the timing works out.
If your second trip is done before the second set of 30 days runs out, and you do your editing fast, yes, you could get both trips on the CD (as long as I haven't done some dreadful math error due to not yet having had even a cup of coffee yet).
But you would want to delay claiming those first pictures, to give yourself the most time.
At the parks, to make sure all the pictures you took are on your account, youc an go to the photopass centers (photo shop on main street in DL, and an area of Greetings from CA in DCA) and have them scan your card, and look on their monitors. Doing this does NOT associate the ID number with your account, so it's safe to do. And if memory serves the monitors are on the large-ish side so it's good to do a quality check of the pix. Make sure the important pictures are there and good, etc. Then take that photopass card home and put it in a safe place (you'll have already written down the card ID number somewhere, maybe taken a clear digital picture of the number, etc, so just in case you lose the card, you still have the *number*, which is the important part).
At about day 29 from the first picture taken, claim the pictures by entering the ID number on your online account.
Then go on your second trip, and as soon as you can, claim those pictures by entering the ID number, madly edit things (you can start editing pictures on your first trip once you claim them, by the way), and make sure you've ordered what you want to order before the second 30 days (from the day you claimed the first ones) runs out.
Once you have that DLR photopass card ID number on your account, you will see DLR pricing. That pricing has been for over a year, $59.95 for the photopass CD, plus $7.00 in standard domestic shipping.