What exactly do you mean by this? To quote from the official 1DX III white paper, detailing the enhancements the Digic X processor has over previous iterations:
"Dedicated sections (“blocks”) of the processor for specific Dual Pixel CMOS AF tasks, and for subject detection (including the new Head Detection AF, and AF tracking capabilities for both viewfinder and Live View shooting)"
This sounds very much like AF processing is a responsibility of the Digic main processor in n the 1DX III. In fact, I guess it does handle the majority of this task, in light of the R6 and R5 AF performance, neither of which has the second processor as far as I can tell.