Unfortunately, the motherboard has only one riser-board slot.For 1 - see if you can move the cards to different pcie slots, you might be drawing too much power or hitting a pcie lane limit. You will need to experiment since it isnt an exact science.
For 2 - nvidia drivers are super picky and it might take a lot of trial and error to get them working. Also you may need to monitor or dummy plug setup on the video card. The other thing i recall is i had to remove the virtual video driver as nvidia drivers looked for that as a clue to running virtual machine. there is alot of info in the threads here so you may need to spend some time reading the threads and looking for what worked for various people and trying it out.
And on the riser there are only three PСI-E ports (two short and one long). The long one takes up the video card and blocks access to the first short slot. And in the second short slot - a wi-fi card is installed.