Remember that they only make a 3 Slot and 4 Slot NVLink SLI Bridge
 

Hadn't actually picked a board; since the cards are the most expensive part, I'm figuring those out first.  All of the options I'm considering certainly have space for the cards, and I'm restricting myself to boards that have the first x16 slot directly next to the CPU which allows for a 4-slot spacing.  (Which as far a I could find isn't even half of the X299 boards and only the two Z390 boards that have a PLX )  My question was only whether using 3-slot spacing was actually a good idea with the FTW3 cards.  After all, they make SLI bridges for 2-slot spacing but it wasn't a particularly good idea to stick two 1080ti FTW3 cards right next to each other.
 
The diagram showing the bridge arrangement only shows double-wide cards, so it... doesn't really help.  If it showed two 2080ti FTW3 cards right next to each other as a recommended configuration then I'd have my answer.  And certainly, 4 slot spacing works to keep a slot between the cards, but then it puts one card right next to the power supply.
 
So: 3 slot spacing puts the cards right next to each other.  4 slot spacing puts one card right next to the power supply.  In both cases, there's not a whole lot of room in front of one of the cards for the fans to pull air.   That's why I'm asking how much space the cards need.
 
If they're really 2.5 slots wide then that leaves about a half slot for airflow.  Is that enough?
 
In case it matters, my chosen case is a Silverstone FT05.  I know a case that supports an 8th slot would resolve the issue completely but the FT05 is pretty much the only larger-than-ATX case that will fit in the space I have available.

There is No 2-Slot Spacing NVLink SLI Bridge as what was made for the older GTX Cards.****
 
RTX Graphics Card Slot Spacing
 Z390 Platform.

 
 X299 Platform.
 

 
**** The Older GTX SLI Bridges.
 
Sorry I am slow to Post.

I'm on 3 slot with 2 2080ti ftw3s, I wouldn't do it on air. They are really a 2.75, you end up with a 1/4" gap for you primary card. Either 4 spacing or go water/hybrid.

I would worry too much about a plx chip, even 8/8 they are basically running 100% as long as you have the nvlink connected.

So now that I'm home and can just take a picture.  Here's my current machine, which I plan on replacing.  This shows two 1080ti FTW3 cards with 4-slot spacing.  It's tough to see but the power supply is about 5mm from the left edge of the motherboard.

So if you imagine 2080ti FTW3 cards in there, you can see that the spacing between the cards should be OK (one slot and change) but the leftmost card will be quite close to the power supply.  I can't really tell how close it will be, though, because I don't have an exact measurement of the width of the card.  It would of course be obvious if I had a card in hand to drop in, but... $1500 is a lot to spend just to see if something fits.
 
It also occurs to me that if the 2080ti FTW3 cards are any longer than the 1080ti FTW3 cards then they'll be too long to fit in the case.  I think I'm just hampered by the internal dimensions of the case, but I don't know of any other case remotely like this one (12"x11" motherboard support, no wasted space on drive bays, no front air intakes, heat rises and exhausts itself even if no fans).  If only they would make one that's just a little bit bigger....
 
I guess I could always just not get FTW3 cards, but it's so hard to settle. Money isn't the limitation here; the space I have available for the computer is.  Liquid cooling probably isn't the answer unless the cards are considerably shorter as a radiator (2x180mm) would need to fit in there.

Well I guess that settles it.  I will rearrange/rebuild furniture and move to a larger case like, I don't know, a Phanteks Evolv or an NXZT H700i and either get 2080ti FTW3 Ultra Hybrid cards or just go full water with Hydro Copper cards.  Which means I need to figure out the pros and cons of having two graphics cards on one loop.  I've done open loop liquid systems before, but not one this big.
 
Thanks for the input, folks.

Thanks for your input, but I wasn't aware that I had requested information on the economic viability of SLI.

epithumia
Well I guess that settles it.  I will rearrange/rebuild furniture and move to a larger case like, I don't know, a Phanteks Evolv or an NXZT H700i and either get 2080ti FTW3 Ultra Hybrid cards or just go full water with Hydro Copper cards.  Which means I need to figure out the pros and cons of having two graphics cards on one loop.  I've done open loop liquid systems before, but not one this big.
 
Thanks for the input, folks.

Pro is they run cooler then having One Loop CPU + 2 GPUs.
I run two myself one for the CPU and one for the GPUs.
I Parallel in SLI and the Temps between the two are less than 3°C
Do you mean a Open Loop as a Custom Loop over a AIO or Sealed Cooler like the Hybrid Cards?

epithumia
I guess this is now sort of off-topic but my concern with a single loop was how the fans are controlled.  One important goal for me is to have zero spinning fans as much of the time as possible.
 
I guess at least some modern enthusiast motherboards have all sorts of inputs for external temperature monitoring so this really shouldn't be an issue with the proper board.  At least the Rampage VI Extreme board does, but that one seems kind of insane.
 
Edit: And yeah, open loop == custom loop; closed loop == AIO.  I've seen both terminologies, and there's at least one product that's sold as an AIO that you can open if you want.  (I used it on an very small 11 liter X299 build I did a while back.)

Yes I have one of them that you can expand it, the Alphacool Eisbaer 360 CPU Water Cooler Fully modular AIO.
They are Only OK IMO.
As far as the Fans you are Talking about the Fans on the Hybrid Cooler or with using a Custom Water Loop?
As far as the Hybrid you use PX1 to manage the VRM Fan and the Radiator Fan.
For "external temperature monitoring" I use AIDA64 Extreme
 
 
As far as off-topic the EVGA Forum Moderator's have stated that if it is your Thread you can take the Topic where you want.
 

This is the First Hybrid Cooler that can also control the Radiator Fan (With PX1).
On the GTX Hybrid Cooler we could only control the VRM Fan with PXOC no control of the Radiator Fan from the Card or Software.
But still cannot control the Pump that comes with the Hybrid Cooler Card or Kit, some would like this but I would keep at 100%.
 
For my Custom Water Loop I use

CTR-CD1224 12V or 24V Pump & Fan Controller With Display (Really Sad they no longer make and sale this unit)
[Replaced By CTR-KSM100]
 
And this  CTR-SPD12X2 Dual 12V Pump Controller [Replaced By CTR-SPD1224M]
 
NZXT Sentry 3 5.4-Inch Touch Screen Fan Controller Cooling (AC-SEN-3-B1)