If you still remember what you have learned from your diving course, the intermediate pressure of the first stage is about 10bar. It means that a balance type first stage will supply 10 bar to second stage no matter how much air remains in the tank.
Will first stage supply air when tank pressure below 10 bar?
The answer is yes.
When tank pressure is below 10 bar, the piston will open freely.
At a result, the tank can be drawn to 0 bar.