Black iron fittings are only good for return line (no pressure) applications. They are not pressure rated for the working side of the system. Use only hydraulic fittings and plumbing for the high pressure side.
That said, you could plumb in a second valve off the return line side of your original valve and it would work fine. Another option would be to buy a 2-spool valve and totally replace your existing valve. A lot of how and why you decide to do it one way or the other depends on funds and parts availability. If you buy a single valve or 2- spool valve, make sure the pressure relief setting is capable of high enough pressure for you to not lose tonnage on your splitting function. If you install a second valve and it has a lower relief setting, make sure you plumb it down stream from your existing splitter valve. If installed before the splitting valve the lower relief valve will pop open first effectively reducing splitting power.
Hope that helps and makes sense