I am not sure about 3 months...
You have 30 days from the first picture to enter the card ID number(s) onto your account. 31 days and they will be lost (exceptions can be made, but it's nerve wracking and not guaranteed). Then you have 30 days to edit them and get the pictures or downloads or CD or products ordered.
If you go to one place, wait the full 30 days and then enter the card ID number(s) from that park, then after that go to the other park inside the 30 day editing period, then they can be on the same CD. But outside of that time I don't think you'd have luck getting everything onto one CD order...rather, the first park's pictures would have expired if you were going to the other park after the 30 days after you entered the first park's pictures were put on the site.