Single gauges have both air bags combined on one circuit. Twin gauges are for control of each side individually.
We all know that air bags don't extend the weight amount that goes on the rear axle. It's really for leveling the vehicle, making it stop and handle better. And you won't get the flashing lights when you spotlight oncoming drivers any longer.
When you're unloaded, you should drop pressure down to 5-10 lbs. Measure at your step bumper prior to loading your trailer. After loaded, inflate the air suspension to the same level and see how it's sitting.
I have a set of Air Ride suspension to install. I can honestly say I've never seen so many nuts and bolts--and bad installation instructions.