The normal method for this is really simple, and still keeps dual circuits, just not cross biased.

1. Take out the prop valve.
2. The front circuit goes from the master to a "tee" which then feeds both front brakes
3. The back circuit (single line from the master) goes to your "pressure limiting prop valve), then to the back of the car, where it too gets a "Tee" which goes to both rear brakes.

The result is, you have two lines teed to the front circuit, and two lines teed to the back circuit of the master with your prop valve in there. Now, test it under threshold braking, and adjust until the rears lock under FULL braking. Once you get that, back it off a touch, and bingo - you are done