a and b nutrients allow hydro formula makers to have stock solutions with higher concentrations without calcium phosphate or calcium sulfate precipitating out..
I honestly can't understand how dynagrow and ionic do the one bottle complete hydroponic formula thing. I have no issues with it, I'm just amazed it actually works and they get no complaints of precipitants. You'd think its shelf life would be very low, and it would be a very unstable product for being one part, complete mix. It makes this thread actually a very good question. Is 2 parts really needed?
Ideally, a and b stock solutions should be designed so that adding equal volumes will yield the intended "complete formula", for convenience.