Bingo!!

I will check the board manual for confirmation.

Reg why I am looking for x8 slot is that all the dual 10gbps cards I found are pcie3 x8. Can I use an x8 card in x4 electronic slot and will it have half max bandwidth ie 4GB/s instead of 8GB/s? or system doesn’t recognize it?

this was suggested in another comment as well. Will check if the bifurcation works only on the hyper card or if other slot can be put in x8 mode.