It would be much easier to just connect the low pressure gauge from the primary regulator to the input on the secondary regulator bank. Dial the primary low pressure gauge up to whatever is higher than you want from your secondary regulators, and you’re done. Easy peasy.
This is the way to go. Also, check the max psi for the secondary regulators. I have one that says don’t exceed 200 psi, so it wouldn’t work being hooked up to the high pressure side. Ask me how I know that… :-[