Kaido is not the strongest living being[/B]. Imu, Dragon, Gorosei, Admirals, Shanks, Linlin... alot of people most likely being above him.
Splitting the Sky is clashing of CoC. We know Kid is able to use CoC. Is quite possible he split the Sky with Kaido even before Luffy met Kaido. But since he is not the MC, it wasnt shown.
Kid even implied he is holding back with law on his side, since he dont trust him.