Tried to use the on board m.2 and could never get anything to see them, so...
Amazon $31.54 with taxes GLOTRENDS PA21 dual M.2 with M-Key. All 3 16x slots are on CPU2. CPU1 has all your 8x slots, your 2- M.2 and every other damn component on it(makes sense if you were to run a single CPU on a 2xCPU board). If you look at the Block Diagram Slot 6 and 7 (far right closest to the DIMM slots) are already bifurcated into 2- 8x slots. I set CPU1 IOU1 to x4x4x4x4 and installed the card in slot 6 (would have used 7 but the fan plug doesn't let a card seat all the way when there is a fan plugged in). I am able to see both of the Intel 2TB and assign them to a pool in Unraid.
Summary (I think):
CPU1- IOU0= Slot 1 plus everything else, also via DMI3( 2x 10gb lan, 2x 1GB lan, BMC, USB, SATA, TPM).
CPU1- IOU1= Slot 6, slot 7
CPU1- IOU2= slot 3 and the 2- 4x m.2 slots.
CPU2- IOU0 slot 2
CPU2- IOU1 slot 5
CPU2- IOU2 slot 4