Combine the best of 2 ISO levels with BRAW

There has to be a way to COMBINE the best of the highlights with a clip set to 100 ISO AND the best of the shadows with the same clip duplicated and set to 1000ISO...??? What say thee?? What am I not considering and why hasn't this been done before??

You just discovered hdr merging, only decades after it was done first.

Simplest way to do it manually is to linearize your footage, scale them so that their exposures match (some reference patch in iso100 matches values in iso1000 image) and then merge the two using maximum operator, optionally also limiting the merge to highlights area, otherwise scaled up high iso will also introduce scaled noise. In the end apply tonemapping as needed to fit this high dynamic range data into output range.

ah! the classic: "my camera has 16 stops, why i cannot use them?"

you can use them, just not AT THE SAME TIME.

the issue you will ALWAYS have is how to fix the boundary between the hi and low exposure, imagine you have a gray ramp from 0 to 100 and you want to get the top of the curve and the bottom of the curve and combine them: what happen at the join?

that is why nobody did it.

Algorithms for high quality merging have been established for long time already.