I ran into this problem once before. We were planning 10 park days, and it was cheaper to buy 2 5 day military salute tickets than a single 10 day ticket (even at the military discount on MYW tickets). While testing/learning the system prior to my 60 day mark, i discovered I could only make 7 days of FPs. Which I couldn’t understand, as if it only recognized one of the tickets, it would only let me book 5 days of FPs. Yet, I couldn’t book for 10 days.
When I called support, I was told that, the maximum number of FP days you can book is the larger of:
1. The number of days on a single ticket
2. The number of days of your Disney resort stay (including check out day); or
3. Seven
Once my 60 day window opened, I was able to make all 10 days of FP.
If I were staying offsite, I would have been able to make only 7 days of FP in advance, and for the final 3 days, I would have had to wait until using 1 days worth of FP before making the 8th day, and so on, similar to those with an AP staying off site (unless Disney customer service pixie dusted me).