I assume it would require having another 'Define Overlay' panel, called, for example, 'Define Overlay2', which could be a duplicate of the current one. I would imagine in most cases, a user would manipulate the separate colorbars/thresholding in each panel. One might make one colorbar have a yellow-red family, and the other with 'cool'/blue colors, and overlaps could sum together in a RGB way, or via opacity?

One method of including the new Overlay2 panel could increase the controller size by 50%, yes. Alternatively, one could envisage the second overlay panel as being a copy of hte current 'Define Overlay' panel, and one could toggle between 'Define Overlay' and 'Define Overlay2', in much the same way the 'Define Overlay' and 'Define Datamode' panels can overlap and retract.

Does that sound feasible, or in the neighborhood of feasible?

Thanks,
pt