can I power two cards with one 8pin - 6+2 & 6+2 cable? just using one PSU

If I do that should I also fill 6 pin slots in both cards?

also https://s13.postimg.org/43sdk42lz/Capture.png is that ok (warning signs and message:

This device cannot find enough free resources that it can use. (Code 12) If you want to use this device, you will need to disable one of the other devices on this system.

?