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?

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

Any valve(ball vavle) 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.