Air gauge, why 2 needles?

OR would be you would have half your brakes

 

Yes, that was helpful.

It also made a lot of sense.

Thank you for your enlightening reply.

 

You're correct - one is primary air and one is secondary air. Primary air provides air to your drive axle brakes, as well as your trailer axle brakes when the foot brake is applied. Secondary air applies air to the brakes on your steer axle, and will provide air to your trailer brakes when the hand valve (Johnson bar) is applied.
This doesn't speak well for your instructor.

 

Your instructor was wrong and you are right it is one needle usually the orange one for primary air and the green one is for secondary air. If it was for the trailer then it should read 0 when your bobtail and it doesn't.

 

Your instructor is an idiot! Once the trailer is hooked up it becomes apart of the tractors air system.

I've never seen a brake application gauge just for the trailer.

 

If one air system failed, you'd still have service brakes to some axles, an the secondary also takes over for the primary in the event of a primary air system failure.
To fully go over the dynamics of it would take me a long time to type out - if you'd like to read the specifics for yourself, then do a search for FMVSS 121, which are the federal regulations regarding vehicle air brake systems. No, if you have a failure in one of the air systems, you will not lose service braking capabilities. However, you need to shut that truck down ASAP if that happens.
As for the Johnson bar, it doesn't bypass anything - it engages the secondary air system. If your secondary air system goes out, it won't function.