No worries, The info is helpful 
I ended up getting the X399 Taichi board myself and when I checked the UEFI all slots supported PCIe bifurcation but only in the x4 variety (so an x16 slot can only be an x16 or an x4/x4/x4/x4)
Was hoping for and x8/x8 option 
I’ve heard of some boards BIOS’s being modified to support it, anyone have any info in that regard?
Thanks for all the replies BTW!
The Asus Zenith extreme x399 can support bifurcation accross all pcie and be x16→x8x8/x4x4x4x4/2x2x… Or the combinations of them to make x16 from reading their product page and bios reviews
That is great to know, thanks for sharing!
if I don’t figure out a way to work around the x4/x4/x4/x4 limitation then I might pick one of those up 
Hi, I’ve been looking for the bifurcation settings on a Gigabyte board…can you tell me where they are? I have an Aorus X399 Gaming (not Xtreme), but hopefully, they are in same place…Thanks!
Bit old thread, but since I have the Taichi, it seems appropriate. Would the ASUS HYPER M.2 x16 PCIe VROC RAID Card work on the Taichi? I do not need the raid, just 4x NVME ssd’s in one 16x slot, so I don’t waste the other slots
Yep, should work - there is an option in the BIOS to set the slot to x4/x4/x4/x4 instead of x16. No other options like x8/x8 as I had originally hoped tho 
OK, thanks. The fact it’s meant for Intel RAID will not stop it from working on AMD boards, but without Raid?
Has anyone had any luck bifurcation TWO x16 slots to x8x8 simultaneously? I have PCIE1 bifurcated to x8x8 and it seems to work fine…but I haven’t been able to get any other slots running bifurcated simultaneously (these are with GPUs). I can set PCIE3 to x8x8 and as long as one gpu is plugged into the splitter (AmeriRack), it posts fine and is recognized in Windows. As soon as a second card is connected, the system won’t post. I’ve tried the x8 slot at x4x4 as an alternative, but it runs into the same issue. Any architectural reasons you couldn’t have two slots bifurcated simultaneously?
So you have two GPU on a riser, or do you mean one GPU is on a riser and one plugged into the mainboard?
Can you share a picture of what you have put together?
Maybe power related?
Do you have Above 4G decoding enabled? might be needed with that many PCIe devices…
Thanks for the tip, I’ll give that a try!
Correct, two GPUs on an AmeriRack x16 to x8x8 riser. In PCIE1 it works great and gets the system up to 5 gpus that run no problem (3 in the mainboard and 2 on a single x8x8 riser). As soon as I bifurcate a second slot and add a second gpu to the riser the system stops posting (oddly, a single gpu on the second x8x8 riser posts). Shouldn’t be a power issue, they’re on a 1500w PSU and I shouldn’t be anywhere near that draw at the moment (many of the gpus only draw 150w max).
You may need to enable above 4g decoding with multiple gpus
Was just thinking, GPU don’t exclusively draw current from the external connector but also the bus. My thought is the riser might not be capable of providing enough current for two GPU, despite whatever it has for its own power connector.
Enable Above 4g Decoding was exactly it! I had to quickly switch my boot drive from mbr to gpt (took less than a minute with mbr2gpt) and the system is posting and Windows is correctly identifying all 6 gpus. Next stop is bifurcation the two x8 slots in x4x4. Thank you for the advice, pretty amazing that the x399 (taichi) seems to be capable of running 8 gpus at x8x8x8x8x4x4x4x4…from a post-production/rendering standpoint, that makes it an absolutely killer system.
Just curious, can you share what you’ve come up with to support the riser and extra cards?
For the x16 riser/bifurcator I’m using the AmeriRack ARC1-PELY423-C5V3 and then I’m just running two Asiahorse PCIE extenders (cheapest I could find, around $12 each). That’s working perfect on the x16 slots. I took a shot with the x8 slots (tried both) and it seems like only one gpu will show up in x4x4 mode. I suspect this may be because the PELY423 is specifically for x16 to x8x8. On their site, AmeriRack mention another model that sounds like it might be specifically for x8 signals, so I’ve emailed them about that and will give it a shot if it sounds like it will work. For a case, I’m using an open air 6 gpu case, but it’s clearly too small for all of this. If I can get the x8 slots split as well then I’ll move everything into an 8 gpu open air case.
Nice, glad you got it working!
How does one obtain these risers exactly?
Do you just have to email Ameri-Rack directly and hope they’ll sell you one?
That was the route I went. There are only a small handful of resellers that I can find and most don’t have stock. But AmeriRack’s customer service was great.
Hello atothen. Where you able to find out if AmeriRack made x8 risers to x4x4? I have a dream project of making a Dual user (Or triple since two cores are left over after two are dedicated to host Esxi) build that can be crammed into a Cerberus x.
Parts list
CPU: Threadripper 1950x 16 cores 48 lanes two 6core vm’s, 2cores for Hypervisor(KVM), Possible 3rd vm with last two cores.
Cooler: NZXT Kraken X53 240mm Front of case
Mobo: ASRock x399 Taichi
RAM: 32GB
Storage: 2x2tb sticks of NVME
Storage: 2x2tb drives of SATA
Graphic Card: 2 rtx 3070’s 
And a hd radeon 6850 sfx black
PSU: Will change but the biggest sfx psu I could find is the silverstone sfx 800 which has voltage instability.
2 Sound Cards
Note: Specific to x399 Taichi
PCIE1 (PCIe 3.0 x16 slot) is used for PCI Express x16 lane width graphics cards. 1st 3070
PCIE2 (PCIe 3.0 x16 slot) is used for PCI Express x8 lane width graphics cards. Two Sound Cards bifurcate using an x8 riser to x4x4
PCIE3 (PCIe 2.0 x1 slot) is used for PCI Express x1 lane width cards.
PCIE4 (PCIe 3.0 x16 slot) is used for PCI Express x16 lane width graphics cards. 2nd 3070
PCIE5 (PCIe 3.0 x16 slot) is used for PCI Express x8 lane width graphics cards. One Nic or an x8 riser to x4x4 for additional x4 lane card.
Does the X399M also support bifurcation? I cannot see any PCIE options in the bios 3.30.