I don't know what state you're in, but you can translate this interpretation:
When you say "legal guardian," I think of something else -- but the way you're saying "legal guardian" and "non-legal guardian," I'm thinking you're talking about PRIMARY CUSTODIAL (PARENT, meaning, the person and address with whom/where the child will hold primary residency) AND NON-CUSTODIAL (with whom/where the child will be when not at his primary residence = the "visiting" parent).
To my knowledge, there is no longer such thing as "joint custody," but rather now "Shared Custody." (Somebody correct me if I'm wrong -- I'm in PA.)
The child does have to have one address to be considered his "permanent address" -- usually the mother's, but not always. Everything else is spelled out in a custody order, which is agreed upon by both parties via custody conciliation, and which you should both get a copy of the order (which should include holiday specifics, other things that can be mutually agreed upon, etc). I think it's great you have decided upon a weekly residing schedule, and I hope you two continue to work together for (and be sensitive to) what is in your son's best interest. In addition, with an agreement such as this, I would pay particular attention to your child's feelings & behaviors, and talk to your child -- or get him into a 'children coping with divorce' type of program. Usually schools have these types of programs. (You haven't said how old the child is, and this matters, developmentally!)
As far as advantages/disadvantages, I don't know of any, as long as you two can get along and work together on this agreement as responsible adults, keeping your son out of your disagreements and/or power trips with your soon to be ex-spouse.
As far as I know, whenever there is a custody agreement between both parents, both parents are to be informed of anything regarding the child when it comes to school, medical, social, etc., ie: equal rights to anything pertaining to the child. This means you both are responsible for telling the other how the child is adjusting, and working together to make sure you do what it takes to make YOUR divorce as smooth a transition for him as possible (And it's not easy!!!)
Now, child "support" (the potential disadvantage of DIVORCE) is a totally different story (and usually two different offices in the courthouse). This is where the 'non-custodial parent' usually gets hit with child support and responsibility of carrying medical insurance on the child (which I'm sure you would do whether you stayed with his mother or not). I'm not gonna go here though...
I will advise, however, not to bring up the WORD "support" in the custody office, and refrain from using the word "custody" in the support office. They are two entirely different matters.
