I've been reading the White Paper and found this tidbit, which explains how the metering sensor helps out with auto focus:

The metering sensor teams up with the new, square pixel AF sensor during viewfinder
shooting, assisting the AF system with its subject detection capabilities. The metering
sensor, with its color recognition capabilities and 400,000 pixel resolution, can recognize an
initial subject, and then update the AF system as to its location during Zone AF, Large Zone
AF, and Auto AF Point Selection shooting through the viewfinder.