Let me reword my question with a scenario:
I go diving at one of the local quarries, I have a plan set for 3 dives throughout the day. On my first dive of the day my Primary fails. I make an ascent using my air2. I return to the picnic tables and remember that I have a spare octo in my save a dive kit. Instead of calling it quits for the day, I replace my failed primary with my spare octo. In this scenario I would always have my air2 as my emergency air supply, I would only be replacing a failed primary with my spare octo.