I see 11.6 has some new Overlay Chart functionality with charting properties available from the Ribbon. Does this allow you to split/pivot both the Base and the Overlay by different data fields?

Bob,

Overlay charts support charting different metrics across the same common axis/legend.  If you try to generate an overlay chart with a non common x-axis you will receive an error message.

I have noticed that if you include a non-common axis in some cases it will just ignore it, and use the axis from the "Base Layer" and not "Layer 1"