This is what I understand about that, though I have never worked on Czech wiring.

I think you have the incoming color code right: black (or brown) is the hot wire, blue is the neutral, and the green/yellow is the ground.

I believe that fixture is set up similar to some USA ones -- designed so you can have 2 switches to turn each set of bulbs on separately, or all of them at once if you want more light.

So if you are wiring it to use only 1 switch for it, you would indeed wire the incoming black wire to both of the black wires from the fixture. And then the blue to blue and green/yellow to green/yellow.

Try that, and see if it works the way you want.

My Czech chandelier has one blue wire, one brown wire, one black wire and one green wire. I know green is the ground, black is hot, blue is neutral. Should I connect the brown wire to my US white wire or US black wire? Thank you