You'll need to make up a tee fitting for the low pressure circuit to check the pressure. I made one of my own by taking an existing dual power tee and tapping it for 1/8" pipe thread.
Once you have the gauge installed, check pressures with the PTO on versus off, and with the dual power in low versus high. Look for a large drop in pressure while shifting. I suspect that the problem is in the high range of your dual power. That's the usual culprit.