I agree wholeheartedly with Heath on this one.
Starting out as a young stage manager, I would often end up as an ASM, and my skills would not be utilized fully. As in - the older SM assumed I didn't know anything because of my age, and did most of my work for me. I soon decided that that the *team* aspect of Stage Management team would be my goal - always.
Whenever I have an ASM, I try to encourage that person to ask questions, help wherever is appropriate, and have an open dialogue with me. You never know what skills people will bring - even if that person has never done this before - they could have been sewing since they were five, or happen to have a background in furniture construction. I *never* assume that my ASM is a subordinate position, because it isn't, and making someone feel subordinate never helps get things done.
I think that in your specific situation - where it's rumored that your ASM might have a chip on their shoulder about being ASM again - it might in fact be *most* helpful to view this as a team. The posting you saw about working "under" someone shouldn't affect you - the ASM doesn't work under the SM, the two work together. Approach your ASM as a team member, where each of you have different but *equally* crucial roles, and it will only help boost your ASM's confidence, and your own. The best SM's work well with people, and if you can work well with this person, knowing there is a potential conflict, you've already gotten pretty far with learning what it is to be a Stage Manager.