You can add the second spouse by adding a spouse. Fill in the new spouses name, vitals and marriage date. It does not effect the relationship to the first spouse.
Hope this helps. Carol
As 'Carol' has already indicated you can add a Second Spouse/Wife/Partner ...
In fact ...
You can add up to a MAXIMUM of 200 Spouses/Wives/Partners to any one individual/person in "Family Tree" of 'FamilySearch'.
Plus, you can also add up to a MAXIMUM of, 100 Parents; and, 400 Children, to any one individual/person in "Family Tree" of 'FamilySearch'.
All that aside ...
Each "Couple" Relationship is separate and distinct from each other.
Each "Couple" Relationship has it's OWN set of Children.
But, that said, Children can have more than one set of Parents, in fact, they can have multiple Parents (all with DIFFERENT "Parent-Child" Relationships); and, be in and across different Families.
Here is a "Knowledge Article" in 'FamilySearch':
How do I add a spouse or partner in Family Tree?
Where it states:
You can add one or more spouses directly from your relative or ancestor's page.
Here are some additional "Knowledge Articles" in 'FamilySearch' that may be helpful:
How do I add step, adopted, and foster parents to a child in Family Tree?
How do I add step, adopted, and foster children to parents in Family Tree?
How do I specify biological, step, adopted, and foster relationships in Family Tree?
I hope this helps.