PCV:Why Pressure Control Valve are Connected in Parallel

The usual reason for such a scheme is that one valve will not fill or pressurize the vessel fast enough during high-flow demands, so you add a second valve. The control system can activate neither, one, the other, or both valves.

Not quite sure if I understand your operation.

If you are using a 1/3 - 2/3 valve arrangement in parallel, the reason is to keep the valve seats from wire drawing when the capacity requirement closes the valve to near 0%. The theory requires the 1/3 valve to open fully and then have the 2/3 valve open for additional capacity. Some programmers open the 1/3 valve fully and when additional capacity is required, they close the 1/3 valve and modulate the 2/3 valve. The point is to keep whichever valve is operating at mid-span or higher to prevent wire drawing.