Sara,
Unfortunately you cannot choose your seats with Promo+ Tickets, only with "Flexible" (full priced) tickets.
Another option for you would be to book the "Mesa 4" or Table for 4 tickets which are 4 seats together, with the configuration of 2 seats facing each other with or without a table in between the seats.
Once you've selected your departure and destination, you will get to the screen with the train schedules. You can check the box "Table" on the very bottom of the "FILTER" ( column to the left of the train schedules).
Unfortunately, there are no more "Tables" available for July 8 ( this is the date you gave on another post).
There are "Tables" available on this route for the month of August and beyond.
Keep in mind that one can book AVE tickets 60- 90 days in advance, and "Tables" do sell out.
I've only traveled with one other person, and when I did book our Promo tickets online, we did get two seats next to each other.
I know this is not a helpful answer to you but hopefully it will benefit someone else.
Enjoy your time in Spain!
P.S. One other option is to call Renfe's Customer Service and ask if you will get 4 seats adjacent to each other when booking Promo or Promo+ tickets.