You will need to invite the client on to the platform which will create an individual relationship with that half of the couple. If both partners (or multiple family members) will be attending individual sessions, they will need to be sent separate invites to their own email addresses for them to accept.
Once the clients have accepted the invites and made sure that there are payment details stored in both of their accounts, you will be able to book their individual sessions for them. We would recommend asking the clients to let you know once they have added payment details to their own accounts.