Air gauge, why 2 needles?

Primary and secondary tanks are connected by a line with a check valve, to keep air from the secondary tank backflowing to the primary, the primary tank is a backup to the secondary runs stuff like air ride, seats, older trucks mirrors and windshield wipers, brakes are supplied by both tanks.

 

Doese this apply too all tractor units?

 

Now iam really confused ?????

 

the Pete i drive has gauges for secondary and primary air as well as gauges for tractor application pressure and trailer application pressure

 

Im thinking it all went to hell when they stopped using toggle switches that you could put those 2'' speckled metal flake extensions on...loved the orange metal flake ones..