Currently, the first name submitted for an event registration is always treated as the host, and any other names are treated as guests. If a non-constituent spouse is the first name submitted, NXT fails to match them and wants to create a new record. If you manually link them to the constituent record, it then adds the spouse as a guest with the same name as the constituent.

For example, Bill Smith (constituent) and Meg Adams (non-constituent) are invited to an event. Meg fills out the RSVP form with P1: Meg Smith and P2: Bill Smith. NXT does not match the submission to a constituent, because there is no constituent named Meg Adams. If I manually link their submission to the record for constituent Bill Smith, the registration processes that Bill Smith is attending and is also bringing a new guest named Bill Smith.