In windows the drivers must be shared between the two nvidia cards 
For this you can enable sli for these offically but if still having issues differrnt sli auto is youre friend it disable the sli check
I’ve tried that and had no luck setting it up, plus far as I can tell the Data Center drivers don’t have SLi support for these cards. I know nothing about adding in support for these cards in Geforce drivers. 
Aren’t all the craft computing setups utilizing vGPU in VMs? Or has he done bare metal?
@FlorianZ im not sure about the different between the the GRID vGPU and non-vGPU drivers and if they contain any major differences, but I believe the 511.65 driver used is a non-vGPU version. It is at least tagged that way as per nvidia’s driver file name convention.
edit: I’m assuming the only difference between the two is the OS that they run on. The vGPU being targeted towards hypervisors vs windows.
Hmmm I should probably buy another P4…. For science right?
Yeah, you got it exactly right. The GRID driver archive typically contains a driver package for the hypervisor, and one for the vGPU guest. The latter can be installed on bare metal to enable WDDM support. At least that’s the NVidia sanctioned way of doing it 
Perhaps your unofficial driver has the licensing disabled? It’s quite a PIA, as NVidia won’t sell you a license for personal use, even if you wanted to shell out the money for one (which would be way too expensive to make sense in a personal setup).
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@FlorianZ I haven’t seen any throttling since I downloaded, was able to set clocks to their rated speeds and everything was stable throughout my evening of testing. Just gonna skirt skirt around that paywall 
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Read what drivers where used ltt dont a video getting two 1060s sli and that video has the kink to the guy who finds the files you need to get sli auto running if the nvdd whatever it is isnt there just search for that file then patch that file and off you go but really you should just need a bridge doesnt the m40 have fingers
If you have a sli hbb link the cards together and see what happens id need to read thriugh the data center bs documentation the other idea would be to mod the drivers to be quadro m6000s 24gb but the fingers shiuld be enabled as you where.able to link these tigether for.comoute workloads ven just try with the tesla driver bone stock and force enable sli through smi
Yes The cards has places fir a bridge, but the outer “front” is not notched and the backplate is also blocking the use of a bridge. 
Yeah looked like yiud need to take the back plate off to find out
Yes I been thinking about doing the mods and removing the backplates, but I would hate if notching out the front cover to allow the bridge to be used F@%k up the air flow with the cards.
I already need a better 40mm fan for one of the cards its hitting 80c at times under full load.
I do Folding at Home with all my cards and that puts a heavy load on them 75c. But Heaven gets them even hotter when benching 80c on the one. The other runs much cooler 65c but has a 15.000 RPM 40mm fan.
Gotta use the blower fans, there’s a larger variant of the optiplex blowers, you won’t need more than 45% pwm
Not enough room in my cases for those and if using the side blower shround I couldn’t run 3x cards or two even. Edit you can see one of my setups here.
as you can see not much room at all to play with.
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Nice job but I think that would still be to long in my case. how long is it?
One of my setups. Official Post your PC guts thread! - #1405 by Abit_Wolf
That is a case fan and its controlled in bios which does a pretty good job if you ramp it up some. HP sucks with fan headers if the fan isn’t connected you get a stop error at boot. Plus there NO extra fan headers anywhere on the MB. Its why you see all those wires I had to use the old HDD power connectors.
I bet you can remove the box and keep the fan
Might can now that I think about it actully its more or less for the back plate of a video card to slide down into like what probably came on your K40m.
Thinking about it more the Z620 could be ordered with a Tesla K40m, but with that fan at full blast and the back plate on the card in the slot I’m not sure that the fan could cool the card. Maybe they have an addon mod for that setup? I’ll have to do a search and see what I can find out. If so It should be pretty cheap since the Z620 is almost out dated.