Good thing I just use a 1080 for primary and a 980Ti for PhysX then :)

@Atonnis: Others have already answered for me, but, yes, I am sure...;)

SLI definitively gives you improved performance in some cases (benchmarks proof that), but the memory is shared; I guess so that each of the cards have access to the same data. AFAIK, it's the same with AMD cards (whatever their equivalent of SLI is).

It's hard to believe, but I have iClone scenes that are able to almost fill the 8GB I have in my current card, so more VRAM is definitely not a luxury anymore.

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Of course, there is always this supposed software that Stardock are working on that will supposedly allow you mash in almost any graphics card you want to improve performance.........

AFAIK it has not happened in the real world yet, but DX12 is supposed to support memory stacking for multi-card configurations like SLI an Crossfire. AMD's "Mantle" is supposed to also be able to. I think either solution is going to require coding for it at the application level to take advantage.

Being that the type of cards that would be used in SLI/Crossfire are getting 6-8 Gig of VRAM nowadays, there may be little incentive for developers to bother. 

When we may see the fruits of this is anyone's guess!

So currently, you get more horsepower, but same memory, like has been mentioned. Do keep an eye on it though, because it may change in the future.

I was thinking more in terms of:

2 Card SLi In Use:
Data for first card - top half calculated and displayed.
Data for second card - bottom half calculated and displayed.

Each cycle requirements for shared texturing/models would be checked.

Thus minimising use on one particular GPU's memory when unnecessary.

Hey, I'm considering logic from the point of view of an IT bod, not a graphics engine/software driver/hardware developer.

I am wondering if/when iClone will get the ability to render in layers, in that sort of setup I am wondering if a card could be set to render all the character animation while another is rendering the terrain etc. And what kind of power supply would you need for two hard working video cards?

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My system: i7 3770 Gtx1060 6GB

4GB GPU + 4GB GPU = 4GB SLI GPUs

You do not get a system that acts like it has "8GB" of VRAM.

You can Google for detailed explanations of "why," but adding cards together does not increase the amount of memory.  I think the short explanation is they each have their own copy of the entire scene, but then coordinate which portion of it (or which frame) they render.  Consider a situation where they render alternate frames, each one needs to hold the entire scene.

There may be scientific software situations with custom drivers that behave differently, but that is the normal behavior for "gaming" systems.

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iClone 7... Character Creator... Substance Designer/Painter... Blender... Audacity...
Desktop (homebuilt) - Windows 10, Ryzen 9 3900x CPU, GTX 1080 GPU (8GB), 32GB RAM, Asus X570 Pro motherboard, 2TB SSD, terabytes of disk space, dual  monitors.
Laptop - Windows 10, MSI GS63VR STEALTH-252, 16GB RAM, GTX 1060 (6GB), 256GB SSD and 1TB HDD

I ask in query, not challenge - are you sure?  From what I've read SLi works by effectively drawing (1/amount of video cards you have in SLi) and thus eliminates the pressure on draw calls.  eg. 2xcards means the cards draw half the screen each drawn at each frame, which make it significantly less.

If that was the case, why would the memory be shared? Surely the SLi controller/bridges would take care of what needs to be allocated to which card?  Or is this just lazy coding by nVidia?

Thanks Atonnis, I think I'll wait until iClone7 is released now. I told myself after getting screwed with video editing programs to never buy one unless they offer a free trial, after buying one from Pinnacle because they had 5.1 audio, and Ulead now Corel, told me they were not going to make a 5.1 audio version in the future, but later did and I switched back. The Pinnacle company didn't offer free trials of there products, but Ulead did. So I bought the 5.1 audio version from Pinnacle, and it worked fine, until you finished the project and if you had a AMD processor or video card, you could not see the menu on the DVD you made, the menu screen was black and if you guessed right where the button was, it played. That was in October, and they didn't have it working until May, but only after you bought a new motherboard and video card. As a few folks said; iClone 7? You still have problems and things to fix with your iClone 6. I know there is no iClone 7 Pro, but as my beef has been that you call this a professional program, but you lowered the quality of your mp4 encoders, then will only let your encoder go only to a "8". Why do I have to use PNG sequential files, because you use low quality mp4 encoder? I cannot accept that with iC7 and it's probably the only thing I need. Other video editing programs are now using H265 encoding, will that be included? Is it in the plans maybe for iClone 7.0, .1 or .2? I'll be just wasting my time and money if it's not. Not everybody needs the same stuff, but I'm sure nobody spends time with details, CC avatars to have a fuzzy video. iClone 6 has given me just about everything I need. I can deal with the buggy final rendering when I have a large project, but that really needs to be fixed without me splitting up my scene. The new bells and whistles are fine, but let's make sure the old stuff is fixed first and works like it's suppose to. I don't really need iClone 7 for lighting, I can tweak the lighting and colors with all the lighting options that was added in iClone 6. But if you fixed or improved the output rendering engines that would make it easier on me. By the time iClone 7.1 may be out I will probably be finished with my movie. I just decided to not sit and wait. Why? Dates for soft or hardware are not written in stone. Bugs? I know about the new stuff, but what has been improved, fixed in the first version of iC7?  For those who don't know the difference in H264 (mp4) vs H265, here it is.  https://www.youtube.com/watch?v=qL22L0mRSDs  Yes I hear the H265 licensing is expensive, but some lower budget video editing programs have it now. If iClone7 is all of that, I won't mind missing the discount for pre-ordering. After my movie is finished, will I even use iClone again? It's not really in my plans. Have a great weekend.    
  

Home built; ASRock X570 Pro 4, AMD Ryzen 9-5950X CPU, AMD RADEON RX6900XT, 16GB video card, 131GB of RAM.

I had a feeling that was going to be the case. Thanks.

Matthew

PC stats:
Intel Core i7-5930K Processor (6-cores, 15MB Cache, Overclocked to 3.9 GHz with Turbo Boost)

Windows 10
Graphics card NVIDIA GeForce GTX 1080 graphics, 8GB
Memory - 16GB Quad Channel  DDR4 at 2133MHz
DirectX - 12

256GB SSD 6Gb/s main HD + 4TB 6kRPM SATA 6Gb/s Storage