A splitter valve is too simple. Like said above, the flow will always take the path of least resistance especially in an open-center control valve system. Open center is the least cost and easiest to plumb. Also, your tractor likely has a single section pump, meaning all hydraulic flow is created by one pump section and you the operator controls where that flow goes.
A priority flow control plumbed between the front and rear control valves could do basically what you are doing with the valve knob you pull. As soon as the front or rear control valve is operated, a signal from the valve being operated would "shift" the priority control to offer full flow and pressure to the operation being used. The priority valve could be plumbed to whichever control is mostly used, so when a signal from the other control section is not at the valve, it would defer to the primary operation. But this also likely add a delay in response to the controls when the priority valve gets the signal and shifts flow to the other circuit. Also, the priority valve would shift back to the other function everytime the secondary function is not powered, like every time you release the secondary function. And it would require another four hoses and connections, possibly also a relief valve between the two circuits. It can get complicated.
If the pump had two sections, one section each to power each front and rear function separately, then either section could be operated at any time even at the same time, as long as the engine could provide enough horsepower. But then each pump section and valve/control section would require fully parallel plumbing. Meaning a separate pressure line from each pump section to the control section, and a return line from the section to the tank, reflief valve, etc.
