I have subsequently found in Atomic documentation that the Z1 should be set up to break at 1.1"-1.4" H2O while the SS1 is speced at 1.2"-1.8". Setting the Z1 at 1.8" with the adjuster out should work for me.
Too much. I set the alternate the same as the primary (with adjuster all the way out) at the least cracking pressure allowed without free flow according to specs. Before I get into the water/jump/whenever, I set the adjuster all the way in for the second stage that is out of my mouth and the adjuster all the way out (easiest breathing) for the second stage I am using in my mouth. I don't differentiate in adjustment between the "primary" or "alternate", neither should free flow with the cracking pressure knob all the way in and breath easiest with the knob out. I actually switch between the primary and alternate underwater occasionally. If I have to share air, I don't want to get any less air or any more effort from the alternate (I give the primary to the OOA diver).