I looked in Rawpedia. Perhaps I missed it but I did not find an explanation of the two tone curve menus in Rawtherapee. I can set either or both as linear, custom, parametric or control cage. A second dropdown menu has options film-like, standard, weighted standard, saturation and value blending, luminance and perceptual.
Blind trial and error seems to show some option combinations don’t do much while others make major appearance adjustments.
Both tone curves effect the appearance of the currently edited image. Do the two tone curves form a composite average somehow?
Rawpedia still needs a work. I’d be happy to help out but…I’m too much a beginner yet. I do love the software.