Think of it like this: You grow to be 18 (or however old Schala is), then lose your entire memory. For the next 18 years you begin to get new memories, but once you reach 36 you regain all your old memories... This is how I think of Schala/Kid.. I don't think their bodies actually 'merge', but that Schala's original body is beat up and ruined, if not lost, from being a part of the TD; however, Kid regains the memories of Schala and thus 'merges' with her..
Ok, now I'm going to get into abstract ideals. I believe that we know everything in our subconscious, maybe not from the moment we're born, but by the time our brain is fully mature physically at least. Whatever happens to us and whatever we learn just takes the knowledge from the subconscious to the conscious and moves something from the conscious back to the subconscious (like not remembering stuff from childhood clearly). Schala, according to this, has certain areas of her subconscious in her conscious, and Kid with other areas. Memories, as far as I'm concerned with this, are just combinations of the knowledge. Although I know nothing technical, in my theorizing I think we know EVERYTHING, including what'll happen in the future, subconsciously. Seeing into the future isn't such a big thing, just as seing tons of gray clouds in the sky and guessing that a storm's coming isn't such a big deal; everything we do forces others to act in certain ways in a giant chain-reaction... Anyway, both Schala's and Kid's memories are in both of their subconsciouses, and once they reunite both areas of thier subconscious are conscious
I guess a lot of that is abstract, hypothetical, and off-topic, but I feel like it'd be a waste for me to have written that and then deleted it <_<