I keep their food dishes on the grate and not under any perches and they use the floor. I also put toys with sticks in them I get sticks from the rodent section and they will play with and chew on them. This way they do use the whole cage. Look for a flight cage,31x21x35.
That's far too small for two, maybe one if s/he was out a lot during the day, but too squishy for two, plus you need to add toys and all which wouldn't leave much room - are they going to be out during the day or be out a few hours a day at all?
In my opinion, I think the HQ Flight Cage is the utmost best cage for a pair of Cockatiels..