putting two needle valves in conjunction will not make them more precise.
The flow will only be as high as the valve that has the lower flow, kind of like only as strong as the weakest link. Whichever valve is set to a higher flow does nothing, because the flow will always be lower due to the other metering valve.
If you have the second needle valve set more closed than the first, pressure will slowly build up between the two metering valves, until it is the same before and after the first valve. After that, the first valve becomes useless and does nothing to the flow of the system, because the second metering valve is the only limiting factor of the flow
good point, didn't think about that.
do someone know c02 density?
there is a formula for this, when two valves setup reach new equilibrium, the flow volume are the same at both valve, but this volume is always smaller than that the single valve with the same orifice/opening.
I remember density, the orifice(cutting surface area of airway), and pressure difference are key elements, but to make sure, I need the book.