I kept my two oscars in a 125 full grown for a few years. I eventually upgraded to a 180. But simply because I could, not because I felt I HAD too.
So I feel you can comfortably keep two full grown oscars in a 72x18x20.
We always want to provide the best home for our fish, you may decide on day to upgrade. But that 125 will do ya for quite a long time. I kept my old man alone for about a year and I always felt he was more active and personable with a partner in his tank hence why I got my second Oscar. My two O’s constantly mouth at each other but do no damage,then will turn around and fin shake and rub against one another (I’m almost certain I have two males.) They sleep in the corner together and are nearly inseparable.