The same time, but only one guy at a time can play the QB position. So much in a passing offense depends on timing, rhythm and chemistry. You lose a lot of that when you rotate QBs.
It sounds great to say you play your best players, but there are very sound reasons why 2 QB systems rarely work.
Of course we stupid fans can debate it all day on CB, but hopefully better minds on the coaching staff will make the right choice.