It MAY be possible-here's how it could work..
If you get your first pictures taken on Sat. Sept. 4th, you have until Oct. 4th to upload them (they'll be available for 30 days from the date taken).
Then you'll have until Nov. 3rd. to order those pictures before they will disappear. For $9.95 you can extend your order time 15 days, until Nov. 18th.
But here's the tricky part-If you are going over TG weekend and will be returning home on Mon. Nov. 29th, you'll need another 15 day extension to get those pictures on the same CD.
I don't know if you can extend your pictures twice-there's nothing on the website about that. I think I may have done a 15 day and then a 7 day extension once on the same CD, can't really remember.
If you can extend a 2nd 15 days then you would have until Dec. 3rd to place your order. If it was me I would call Disney Photopass and ask them if you can extend your dates twice. If they say yes, then for $20 you can get both trips on one CD.
If you get a yes answer, I would make a note of who you spoke to and when, just in case.
I hope it works for you.

If you find out you can do 2 extensions could you post it here? I'm sure there are others who would like to know.
I've done many PP CDs and just love them! I've been able to combine WDW and DLR on one CD several times. Some with just our family, other times with friends.
Have 2 great trips!