Greetings.
As I expected, my formula for calculating main and offhand attack values were ignoring any formulas you'd set in BaseParamControl, instead using the default formula (base + plus) * paramRate * buffRate + flat. I corrected this issue by using the real base value in my plugin, and then removing the attack values of the opposite weapon, rather than calculate an entirely new value based on the default formula.
I am not 100% sure that this will completely work in your specific case, though, as the mat of equipped weapons doesn't play into the mainhand/offhand calculation, so it may end up giving the mat bonus from both weapons on both the main and offhand hit. I did use your example formula, though I didn't reduce atk to 0, and it did appear to split correctly though, so let me know how that goes.
You can download the updated version of the plugin from the main store page, or from this devlog.
-Ramza