The Wikia page (as reliable as it can be) on Tatsuya says his original Magic Calculation Area is completely taken by his Decomposition and Regrowth magic.
Tatsuya can only freely wield two types of active magic, based on his innate ability of directly interfering with structural information; Decomposition and Regrowth. [...]. Owing to this circumstance, his natural Magic Calculation Area is dominated by these two powerful abilities, making him unable to wield other magic.
It also says that the magical surgery performed by his mother implanted another Magic Calculation Area in his limbic system (part of the conscious mind).
In order to allow him to use Systematic Magic, instead of merely his two natural "powers," his mother and her twin sister operated on Tatsuya when he was 6 years old, installing another Magic Calculation Area or Magic Processor (resulting in him having two) in his mind which allows him to use magic.
My question is, do the light novels really state that he has two Magic Calculation Areas?