timo Have a look at Day One. I had multiple diaries when I was with them, a lot of your questions, or thoughts will be answered, for example:
timo how to treat the app settings? Do they apply to both journals equally or would there have to be a separate "settings" instance for?
"One" settings, universal across all diaries.
timo What about the reminder notifications? Do they trigger for both journals or only the "active" one
again, one reminder for how ever many diaries you have. Its then up to the user which diary/journal they update. Users can choose to also have the option of displaying all diaries in the timeline view or just the main one.
timo Does both journals always sync to the same cloud account or can they sync to different ones (so users could have a private journal and one they can share with others)?
All diaries to same cloud account, but in settings, you can mark a particular diary as NOT E2E encryption therefore it would be accessible via the web version for example.
Essentially, what I remember is that all the settings are global and the only "separate settings" would be toggle switches for which diaries to show in timeline view, make E2E encrypted, or 'do not show in timeline' etc etc