Made some research. Double GPU cards like R9 295x2 are pulling up to 300W from 8pins wire, that has 3 circuits in it.
This gives 100W per circuit.

Corsair AX1200i can output up to even 480W on 8pins wire (40A) -> what gives even 160W per circuit.
Problem is i have different PSU, which has only 8+6 pins wires, like engeneers wanted to say that 300W is really max PSU can give on 3 circuits, though only 225W is safe.

So 8pin to 2x 8pin splitter seems reasonable. But why market is flooded with 6pins -> 2x 8pins. (Why even bothering doing them?)