I asked a similar question and later found this answer:
You have 2 posibilities:
1) Need to manually add members from smaller tree to the bigger one. I recommend you to have both trees opened on 2 tabs of the browser.
2) You can try to export both trees into GEDCOM, merge them (search on google for gedcom merge tree) and reimport as new tree, add pictures again... (You will get a little bit messy)
answered Apr 19, 2020 by Jaff (3,460 points)
I hope this helps.