IMO it depends on your level of fitness. I know a lot of people who do a 3 on/1 off split and get great results. I do 4 and 1 myself right now simply because I just can't seem to fit shoulders in with anything else.

But yes, I think you can do back twice a week, especially when 'on'. I know you said you just hit PCT but you should still be able to go 2x a week. The best thing is to give it a shot, see how you feel, and adjust accordingly. If you went Mon/Fri and felt good on both days you could keep it up. If you feel over trained you may want to go every 5 days, so it would be Mon/Fri, then Tues/Sat the next week, etc.