We have trouble with a R2UU-2E8 in an X8SIU-F and two add-on cards.
Each time we put in both cards together, the bottom one is not recognized any more.
Only one card, no matter in which slot, works fine.
When using 2 cards with the riser option for X8SUI(-F) you have to set jumper J32 to position 2-3. When using 1x PCIe x16 cards then this jumper has to stay in position 1-2 (default).