had a slightly different experience - was trying 4 Y return tickets from JFK - TA was saying no availability (had the same thing when i checked online) and then tried booked them separately 2 + 2 and it went through with the same fare class for both the PNRs

dont know enough about airline ticketing to understand but i suspected at the time that i got lucky and the seats opened up when 2+2 was being tried so i thought it was a matter of extremely good timing