I believe since rather than saying "Two Handed," and instead using the verbiage of "Mode," the Scythe is intended to only be used in one hand, and only in one mode at a time. So at the top of a round, you would decide whether to use it in Reap mode or Reave mode.
However, I do believe that this could use some clarification, because the very definition of Versatile in the rulebook indicates that this term essentially means "can be used in one or two hands." But I would still default to my interpretation above since specific rules within realms override the rulebook.