Rather than thinking it of one paid ticket and one free ticket, we look at it as two tickets with a 50% discount.
Let's say the scenario is you wish to create an access + companion ticket as 'buy the access ticket and get the companion in free.'
If the seating is not allocated (is General Admission) you could, for example, create an access + companion book type:
Administration Module > Setup > Sales > Book Types > New
Enter ID > OK
Enter Description > OK
What you are going to do is effectively have one price for 2 seats, so you need to make the Booking Min 2 and Booking Max 2 so the patron must buy 2 seats.
The pricing will need to reflect that of one access ticket divided across the two seats. So if an access ticket is $20 then the access + companion book type will need to be $10:
If you are creating a scenario such as this, make sure you have an access ticket that is also able to be purchased singularly (without companion.)
Related articles:
Troubleshooting: Why is the book type showing in Box Office and not online?
Troubleshooting: A Book Type is not appearing on the Booking Skin