The MB must be some SLI/CrossFire board

one 4gb card make on Pcie 3.0 x16 (works as x16) gen3 slot now ~26Mh/s
Same board 2pc 4gb card on 2x pcie 3.0 x16 (works as 2x x8) gen3 slot makes 20-21Mh/s per card

For me was asus z170/z270 boards the best solutions