You have to use one master cylinder to make it work, with the bias bar you have to decide what end of the mower you won't have brakes on. With a dual set up as soon as one master is through its travel there isn't enough mechanical advantage there to operate the other master. So there isn't enough pressure there to restrict with the adjustable valve to begin with.
I have so much line pressure with one master I can lock up my front wheels on pavement. I have them adjusted with the valve almost shutting the front brakes off completely and I can still lock them up on loose tracks that way. As a matter of fact, I have a valve inline that I can shut the front brakes off completely if I can't back them off enough, that has happenned on wet grass tracks until the track gets worked in a little.