To control the flow, you have to use a bypass valve, not a restrictor. That means there must be a way for the excess oil to return to sump, either through a third line, or teed into the return line.

I set up a planter years ago for a customer on his open centre hydraulic system. In order to run both the motor and have oil for the planter lift and markers, we hooked into the port on the tractor where you would connect a loader. The bypass oil went back to supply the remotes and 3ph (same as the loader return line would be). The motor return line was connected to sump so there wouldn't be back pressure on the motor.

This sounds simple, but there is one catch. If the oil lines unhooked (as is known to happen!), the pump flow on that tractor would have deadheaded (we were teed in before the relief valve) destroying the pump. We had to put a relief valve into the circuit just prior to the coupler for the motor pressure line.

Another problem was the motor was turning all the time the tractor was running. If you wanted to unhook the planter, you had to connect the motor supply line to the return line on the tractor to get oil to the valve stack.

You have the majority of the cost of a pto pump involved to make an open centre tractor run a motor circuit and a lift circuit at the same time. The basic instructions on how to do this were available from White at the time, and likely most other manufacturers.

This customer wanted the ability to unhook one tractor and put one of his closed centre tractors on the planter in a pinch. He didn't want a pto pump. That is why we went to the trouble. We had a neat system, but I still would recommend a pto pump instead.