Why two gauges on a leak down tester?
 

Why cant you set the pressure to 100 psi and hook her up? The reading you get is the inverse of the leakdown, no? What am I missing here?

I believe your two gauges are for the following:
gauge 1 - to measure supply pressure
gauge 2 - to measure leakdown pressure

Leakdown is calculated as such:

leakdown% = 100% - [(measured/supplied)*100%]

5% = 100% - [(95/100)*100%]

I think this is how is goes, but others may know better.:D

Why not install a valve after the guage and simply close to check supply pressure and open to check leakdown?

At first glance, you'd think that would work.
I believe the dual gauge leakdown setup has an orifice such that the supply air is restricted from entering the cylinder (i.e. the supply air is carefully metered into the cylinder). Without this, you'd have to have huge problems to see any leakdown. When the air flow into the cylinder is metered by the small orifice, it is more sensitive (i.e. sees easier) issues such as worn rings or valve seats.

So in a nutshell, you have the supply air that is directly measured with a gauge. The second gauge that measures the "leakdown" is isolated from the supply by a metering orifice. If the air leaks away from the cylinder faster than the metering orifice can keep it at the supply pressure, then you have leakdown to some extent.

Make any sense? This is my understanding of why you need the two gauges and how they work. Again, maybe someone out there knows better.:D