In my opinion i think Zetsu is split into two personalities in the manga and that is why i think he has two bodies in one.
First of all when he always talks to himself but as you see in the manga the font changes everytime a different side talks.
Second in the Itachi Vs Sasuke battle the dark side is always explaining to the light side about the history and legend and all the jutsu's they're using so i think the dark side has the wider knowledge.
So to sum it down i think Zetsu is two different people and were brothers who either were born in one or merged together later on. I think the dark side is the older one and the light side is the younger one as the light side usually is clueless about most the things in battles.