you can - but you need to be careful of your "semantics"
truth is - all billion records in FS are all in one single database
(one humungous family tree)
the family trees are not really "partitioned" as you imply.
BUT to start out a new set of records - for a person who is not already in FS
click on "Family Tree" / "Person" / "Recents" / "Add unconnected Person"
but always remember - this is a single collaborative database.
what you view as "your tree' and what I view "as my tree' are merely "perceptions'
in reality it is all one single family tree database.
Just use the add spouse feature and go from there. The site supports multiple spouses
@PatriciaSullivan35 PatriciaSullivan35 Add him as a spouse to your mother. As KathySexton1 said, FamilySearch supports multiple relationships. Then you can build his ancestry out from there. Let us know if you need more assistance.
Enjoy the journey!