niche' ROMED4ID-2T PCIE Ports usecase

Hello all together i hope i can get an insight in what iam doing wrong. :D

I have an ROMED4ID-2T board and an Epyc CPU Rome in it, i have an 1U case and want my GPU to be seated in the "front" since my PCIE Slot7 (Pcie 16x Slot) is occupied by my Mellanox 100Gbe NIC i want to adapt 2 of those SlimSAS (PCIe 4.0 x8) Ports to one PCIe 16x Slot.

AFAIK the Slim Ports 5+6 are linked together by PCIE lanes Pcie G1 0-7, 8-16

so i bought good cables and two of those SlimSAS PCIe gen4 Device Adapter 2* 8i to x16 daughter boards.

so what works and what not, both do 1x8 Mode no problemo but not 16x Mode.

I tried to interchange the cables but no 16x, i tried every bifurcation mode in the Bios.

Maybe i have to change something else? Has anybody tried something similar.

regards Dom