You'll need to do this via the call centre or a travel agent.
They will sell 2 seats under the same booking.
Second ticket will have the name LASTNAME/EXST
Don't try to book this online as LASTNAME/EXST might be flagged as a no-show passenger and the seat given to another passenger if the flight is full.
When checking in, you'll need to go to the counter as there is a special way to check in LASTNAME/EXST so that this "fictional passenger" is not included in the passenger count on the flight manifest while at the same time blocking the seat from being assigned to anyone else.