This is a typical installation of the dual stage air-assist in the Nova series machines. Your configuration may differ slightly.

This is the back side of the main air inlet on the rear of the laser. Line #1 is supplied from the compressor at the inlet and travels to the solenoid bank.

At the solenoid bank, #1 splits and goes into the inlets of the 2 identical air solenoids (these are normally closed valves). These solenoids control the raw air inlet to the stages. Line #2 is the low volume stage and line #3 is the high volume stage.

Then lines #2 and #3 go to their respective needle valves. the 2 stages exit the valves and combine back into one line, line #4.

#4 runs to the nozzle

The TL Timer board controls these functions: Nova Series DSP TL Timer V8 Block Diagram

The dual-stage air-assist retrofit installation guide contains all the parts, wiring diagrams, etc... of the dual stage air system: Retrofitting Dual Air Assist

Here is more on air: