I made one application to cover both my husband and me plus one statutory declaration (by me). At point 2 of the statutory declaration I made a brief statement that the application covered us both, we were leaving for compelling reasons, that we intended to be absent for at least three months and that further information was provided as an addendum to the statutory declaration. The further information was 2 pages describing our circumstances and reasons for needing to travel, travel history (to show we regularly spend lengthy periods in the UK), work and financial situation (retired), family health issues etc, and listing the attachments.
Then I attached copies of our birth certificates, UK and Australian passports, marriage certificate (all to establish family connections), parents' passports (as evidence of age), tickets to and from the UK, several bank statements (to evidence we can support ourselves overseas indefinitely), vaccination proof and a couple of other documents specific to the UK family situation.
Incidentally, it doesn't say that you shouldn't book tickets until you have received your approval until after you submit the application. You just need a proposed itinerary if you haven't got tickets.
Good luck with your application!