the first card works fine..
but the second card i can't see in the device manager.. do i have to change someting in bios?
on the second card there are 2 plugs.. one with 6 pins.. and another beside of it.. with 8 pins..
i plugged both with the suitable cable..
my second power supply is corssair vs650 - this i tried to use with the second card
power supply works fine - i have done this test
Just so we don't give too many redundant suggestions, what else have you tried?
Have you tested each card independently (directly into the x16 slot) to confirm that both cards are in working condition?
Have you tested the riser? (with a single card into the x16 slot)
Have you tried using dummy plugs or attaching a monitor to the second card? (I don't think this would be your problem, but I haven't used Win10 for mining before.)
On my Asrock H81 Pro BTC, sometimes I've had to clear CMOS to get a card to be recognized. Have you tried this?