The 20 in hg is good for normal engine vacuum. But it sounds like you have the vacuum lines (at least one) tapped in the wrong place. The dual gauge works great if connected properly. When boost kicks in the gauge will shift to the boost area indication. With no boost show regular engine vacuum.
The vacuum line running from the top of the supercharger into the firewall and HVAC system is accessory vacuum not MAP. The MAP line runs from the front area of the SC (under nose drive) and back around to the MAP sensor located behind the supercharger. This is where you make the tap in for SC boost.
For regular engine vacuum you can use any line off the supercharger like to the fuel pressure regulator or boost valve, etc.
See link below for boost gauge hookup and where you tap in:
http://www.grandprix.net/upgrades/boostgauge.html