"Chad Lemmen" wrote
With one or both engines running at least 1700 RPM I get 5" of suction
and with both engines running at 1000 RPM I get 3.5" of suction, but with
only one engine running at 1000 RPM I get around 2" of suction.

First thing to check is that both regulators are closing completely at low
rpm.
Simple way to check is to temporarily cover their relief intake screens with
duct tape. If this increases vacuum at idle, that's the answer.
If the regulators haven't been disassembled and cleaned in years, then
accumulated dust/gunk around the regulator's valve seat can prevent it from
sealing shut. Remove the regulator, tap the wrist pin out to disassemble,
give it a good scrubbing in Stoddard solvent, dry, reassemble and reinstall.
Valve should then close and seal completely at idle.

Second thing is to check all vac hoses and fittings for cracks. Rubber
hoses have a nominal 10 year life. Nylon fittings get brittle with age, and
become prone to cracking after perhaps 20 years.