Some fans need to stop overrating splitting the sky. It just means you have two strong will individuals clashing with ACoC.
Kaido and Big Mom split the sky when they fought. On the rooftop before they performed hakai Kaido asked Big Mom and if she still had the power needed to perform the technique. Think about it, Kaido wasn't sure if a person who could split the sky while clashing against him had the power to perform hakai.
Either, hakai is it technique that require greater amount of power then the amount Kaido and Big Mom used when they split the sky and Zoro defensive power is greater than 2 Yonkos combined power to split the sky. Or splitting sky it's not a way to rank and determine characters power level because it is just a clash of will power.