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: https://support.thunderlaserusa.com/support/solutions/folders/65000060154
Here is a video overview and a note about using 2 different air sources (stock pump for low stage and an external compressor for high stage: