This is my theory.... Remember even during the campaigns, Ruto was more energetic while Uhuru often appeared towards the end of the day. In other words, Uhuru was lazy and he carried on the same attitude even as president. Either he delegated more activities to Ruto or Ruto was too enthusiastic seemingly overshadowing his boss. If it is to be taken as fact, that is why, Ruto says that Uhuru did not want a Uhuruto second term. He wanted a Uhuru second term so he could secure his legacy.
Thus Ruto was pushed to the periphery. Uhuru did a handshake. The handshake partners came up with BBI. Ruto says he was involved in selecting the panel. Then somewhere a long the line he realized that the BBI was a ploy for Uhuru to be in government beyond his constitutional mandate effectively locking him out of the presidency.
That is when the visible split must have happened.
I wonder, what would have happened if Ruto went along with the BBI shenanigans and somehow it went through thus restructuring government? How would that have affected his presidential ambitions in the eyes of Kenyans who were suffering economic and health hardships at the time?