Phil, as others have said, I think you will discover that the line you have labelled as "return" line is infact the by-pass/pressure relief valve line; the relief valve is probably at the connection to the tank.

It appears to me that the oil from the tank flows through the valve body in an inlet channel directly to the pump.  The pressure from the pump is channeled to the valve where it is directed either to the pressure outlet (when raising the box) or back to the inlet channel of the pump (when the box is in "hold")  When the box is in hold the valve blocks the flow to/from the outlet port.  When the box is lowered, the valve directs the return flow to the inlet channel which returns it to the tank.

Locating that relief valve is probably critical to your project.  You should be able to plumb your splitter valve into the pressure line; it appears you will need to route the fill auger hydraulic motor return line directly to the tank.

I have discovered it is best to leave the splitter set to flow the pressure oil through the fill auger hydraulics; the electric over hydraulic valve will allow the flow to circulate harmlessly back to the tank if you are not running the auger.  It can be get confusing when you are using the auger and then switch the splitter because the box will raise unless you switch the lift valve to "hold" or down.  It probably would be best to disengage the pto before changing the splitter; at least stay sober :-)    BE CAREFULL!   Gary