Hello there this may be a repeat question just finshed putting together my latest build have a few problems my system a x299 dark , I9 7840 Intel cpu , corsair ram 32 gb and 2 intel 750 NVMe PCIe 3.0 x4 2.5 U.2 ssd 800 gb . ok here is my problem when setting up the drives only one will be showen after windows is installed i can select the drives in the onboard device configuration but it does not show up in the NVMe info page i do not want a raid system want to use both together looking for help in setting up my bios config to take full avantage of these drive any help is much app .

KPWie
Hello there this may be a repeat question just finshed putting together my latest build have a few problems my system a x299 dark , I9 7840 Intel cpu , corsair ram 32 gb and 2 intel 750 NVMe PCIe 3.0 x4 2.5 U.2 ssd 800 gb . ok here is my problem when setting up the drives only one will be showen after windows is installed i can select the drives in the onboard device configuration but it does not show up in the NVMe info page i do not want a raid system want to use both together looking for help in setting up my bios config to take full avantage of these drive any help is much app .

Fist - exactly which CPU do you have?? Did you mean i9-7940x ?
 
Look at page 28 of the PDF X299 Dark Manual
 

PCIe Lane Distribution (Core™ i9-79xx, 44 Lane Processors)
 U.2(PU1) x4 PCIe Gen 3 lanes from CPU
o Must be enabled in BIOS, which disables M.2 110mm
 U.2(PU2) x4 PCIe Gen 3 lanes from CPU
 M.2 Key-M (110mm) PM1 – x4 PCIe Gen 3 lanes from CPU
o Supports PCIe/NVMe only
o Must be enabled in BIOS, which disables U.2(PU1)
 M.2 Key-M (80mm) PM2 – x4 PCIe Gen 3 lanes CPU or PCH based on configuration.
o Supports Optane™/PCIe/NVMe
o If PE6 is populated by a video card (anything using x8 lanes) this receives x4 lanes from PCH, otherwise x4 lanes from the CPU are used when PE6 is empty.
 M.2 Key-E (32mm) PE – x1 PCH lane
 
PCIe Lane Distribution (Core™ i7-78xx, 28 Lane Processors)
 U.2(PU1) x4 PCIe Gen 3 lanes
o Must be enabled in BIOS, which disables M.2 110mm
 U.2(PU2) (44 Lane CPU ONLY)
 M.2 Key-M (110mm) PM1 – x4 PCIe Gen 3 lanes from CPU
o Supports PCIe/NVMe only
o Must be enabled in BIOS, which disables U.2(PU1)
 M.2 Key-M (80mm) PM2 – x4 PCIe Gen 3 lanes from PCH
o Supports Optane™/PCIe/NVMe
o Must be enabled in BIOS, which disables PE6
 M.2 Key-E (32mm) – x1 PCH lane

 
 
When you enable the U.2 PU1 port it disables the M.2 PM1 110mm port
If you want to utilize both of the M.2 slots on the Dark and use 2 or more U.2 drives you would need to get an PCIe slot U.2 adapter card
 
TLDR: With both M.2 drives active you can only use the 2nd U.2 PU2 port