I was running one 2080ti ftw3 perfectly fine. Using as a CUDA compute machine. Temps never got above 70C even training taxing NN such as StyleGAN. Was amazing.
 
I recently added another 2080ti ftw3 directly below it. I can imagine this is sub-optimal but I have no other options. So with 6 slots, the top 3 are occupied by one 2080ti ftw3 and the 3 directly below are occupied by another 2080ti ftw3. There is very little space between the cards. I can get a pic if needed but just moved the machine under the desk to abate the fan noise and that is not immediately available. I understand this is the consequence of motherboard and case choice.
 
Under load, the top card is reporting P2, 88% fan, 88C. the bottom card is reporting P2, 63% fan, 67C. My EVGA CPU water cooler is now showing red LED on the logo which never happened with one card presumably because internal case temps have bumped the CPUs over 45C. This is a Linux machine so running "sensors" shows CPU cores 55C and below.
 
That NVLink is offered on 2080ti cards at 3 spacings, I'm presuming I am okay. But just asking the community: is this setup sustainable? Should I have gone with rear exhaust options? This is my first GPU compute effort and I appreciate any input or thoughts. Thanks in advance.
 
 
 
 

Looks like your top GPU is almost at the threshold of throttling and possibly shutdown @ 89c which is the max temp for these GPUs. 

If budget permits and is possible with the case, I would see to get a hybrid kit for both to help keep those temps down, assuming your air flow is up to par internally.

EVGA sells a 4 slot nvlink connector. Of course your motherboard spacing needs to match.

Hybrid cooler for the top card atleast! Bottom card is always cooler then the top.. but I guess why loose the fun with the overclocking potential of just one card running cool when the other would hold it all back. Dual hybrid coolers.. guess you should be able to afford it if ya already running 2x 2080ti cards.

Edit: prob be easier to just do it right and go all out and do dual water blocks and slap a quad 120 rad or dual tripple 120 rads.. if ya got the room.

Thanks all for your input. I do appreciate it. Looked in to extender ribbons (thanks again) and adding additional fans.
Turns out a bios update dramatically increased performance, lowered temps, and ceased the intermittent reboots under GPU load that I had attributed to thermal conditions given GPU proximity.
 After the BIOS update under the same conditions, I see top card at 75C and bottom card at 55C and they are totally stable.

Sorry.. yeah best thing for you is a custom loop, evga blocks for both.

Or you may be able to add just 1 hybrid cooler to the top card.

this really isn't sustainable, you need to atleast buy a hybrid cooler and quite possibly a power supply. definitely use one cape per input as BC said.
 
75c should really be the max you let them see

All, thanks for your valuable input and ehupp01 sorry for the late reply on the BIOS update:
 
FYI: I have a Gigabyte X299 UD4 motherboard which I purchased around Jan 2019 with a BIOS version F4. My CPU (Intel(R) Core(TM) i7-9800X CPU @ 3.80GHz) purchased at the same time isn't supported until BIOS version F6j according to the link below:
 

 
I updated to F6j and now have a much more stable setup and much cooler temps (top card at 75C and bottom card at 55C) as reported earlier. The update didn't take with both GPU's in, I took them out and had a single spare GT1030 that worked for the BIOS upgrade for some reason. Once the BIOS updated I swapped the GPUs back out and all was good. Dunno what that was all about.
 
I've also accepted the advice of commenters and used separate PCIe cables for each 8pin connection. Seems like a best practice.
 
After the BIOS update, temps and noise are much less of an issue given exact same tasks, and I'm looking in to EVGA HYBRID Kit for EVGA GeForce RTX 2080 Ti FTW3, 400-HY-1484-B1, RGB for the top card to further redice temps and sound. But paying a $300 premium for the FTW3 cooling system and THEN paying $170 to throw all that away and just do hybrid two months later as if I had just bought a FE is a bitter pill to swallow. Honestly, I think some nominal price break would be fair. But ultimately the purchase was my call and I'll take the hit maybe.
 
 

cudadev
 
But I think I finally have a pretty awesome, stable, and very high end system for work and school. Many thanks to EVGA support and the EVGA community for guiding me through my first build. Much appreciated.
 
Next stop: cable management :)
 

Glad it worked out. 

I will say, web of cables and in small cases aren't SLI's best fwend.