Multiple parts, at best, means multiple zips that you will have to download, one by one in their entirety, to play the game once you are finished. PAR files, essentially, for those who are old enough to remember those, or Rxx files if you ever used RAR.
Separating the game into multiple single-character stories is simply unfeasible, partly because character stories are intertwined, which means you *will* need additional characters to continue a given character's storyline, but mostly because of the sheer amount of work it would entail.