Only individual guests can be assigned specific seats in a table (assigned seating), meaning that plus ones must be entered as separate guests to allow for assigned seating.
Couples can be assigned together to table as a couple, but not to specific (assigned) seats.
Read this article for more information on how you assign specific seats to a guest:
https://allseated.zendesk.com/hc/en-us/articles/360011979120-Assign-Guests-to-Specific-Seats
