Could somebody explain (if possible) how a two-stage pressure-reducing valve (PCV) can allow to avoid choked flow through it, when upstream pressure is 10000 mbarg (10 barg) and downstream pressure is 50 mbarg?

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The simplest explanation is that it won't, If I have got the theory right.

Any valve will have a max capacity given by the Cv and the driving force given by the pressure difference before and after the valve.

If the pressure difference is large enough, the valve will choke. (Choke: max velocity reached)

I believe you are talking about gas.

If you, however, are talking about liquid water, cavitation will occur, but the valve construction will allow for this, formed to accept or send cavitation to areas not (much) affected by gas-bubbels collapsing.

The purpose of a 2 stage pressure regulator depends on the valve manufacturer and the size of the valve.

In this case the flow will be choked, however the use of multiple stages is not to prevent choked flow, but either (as stated, depending on the valve and pipe size) to reduce noise and vibration as a result of high energy levels in the valve. If the valve size is small (IE less than 0.5") then the outlet velocities of the fluid maybe high, thus a multi stage trim in the valve, would reduce noise and be able to absorb a higher amount of energy.

I would suggest that your downstream piping was designed to keep the velocity of the fluid in your pipe within acceptable limits... so avoid making the pipework smaller than the valve.

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