This is tricky

yes, if you have 6-Pin risers then you can power it with a 6-pin directly from the PSU. 6-pin risers are the best quality for risers. Better than molex or Sata risers.

Running 2 risers on one cable is ok, as I am currently doing it for 2 months without problems. PCI slots usually provide about 75W to a cards, rest of power is taken from the 6/8pin plugs to the cards.

Depending on what cards your are using and if you are undervolting to limit power, you can run 2 cards on one cable (2 x 6/8 pin). I do this with my GTX 1060's running 2 cards on one cable 2x 6 pin basically daisy chained.
With lower power usage you can do this.
With cards drawing more power i would only run one cable from PSU per card, as you don't want to overload a rail. That 6/8 pin cable can only provide so many Amps, so even when you are staying under the PSU recommended wattage, it is possible to overload that cable if the cards draw too much power. In your case without initial knowledge of undervolting, rather run one cable per card. don't put 2 cards on one cable.

I can tell you that it does work on GTX1060's drawing 85W undervolted from my Corsair 1000W PSU with one cable supplying 2 cards. I also power 2 Risers per PSU cable via SATA with no problems. running 2 months like that now. No more than 2 risers per cable though! if you can and have the cables do one riser per cable for safety. specially if you have cards drawing more power and you are not undervolting!