Hi,
You're correct about the proportioning valve. But, is the brake-system of a bug x-split? I don't think so. The MC has 3 outlets. 2 are for the front wheels and the other one goes to the rear where it is split for left and right. This way it is split front and rear.
Place your proportioning valve in the line that goes to the rear, so you can change the bias going to the rearwheels.
This is mostly done in racing during wet conditions to prevent rearwheel-lockup.
I wouldn't recommend it for street use unless you can lock your proportioning valve after you found your desired setting after you fitted those huge six-pot calipers front and rear
.
Not being able to lock your valve can cause front or rearwheel lock-up when somebody changed the setting of the proportioning valve without you knowing it.
Sorry to be so long but I felt it necessary. A proportioning valve is great to have but can be dangerous as well.
Hope to have helped (a little)
Cheers,
Richard