Hybrids are a complete waste of time and money as they bring nothing to the table.... the EVGA SC series has been problematic since the 5xx series, and this time the FTWs design was deficient also as peeps cards were getting black screens and some even went on fire. EVGA corrected the problem, if you get one of these pre-November made cards, you can get a thermal pad kit from EVGA to correct the problem.
It must be recognized that Boost 3 is a limiting factor on all but the FE cards which are limited by throttling. The reason most folks water cool a GFX card is to get more performance. So which card will perform better ?
User installed EK Water Block @ 39C GPU temp / 50C VRM temp
MSI preinstalled Seahawk model w/ EK Full cover water block @ 39C / 50C VRM temp
Hybrid Cooler w/ GPU water cooling and stock level VRM / memory cooling @ 50C GPU temp / 70C VRM temp
AIB Air Cooler @ 72C GPU temp / 70C VRM temp
The thing is, a card will perform at optimal speed up to a point which the card starts to throttle at 82C. So all of the cards will perform the same as all are under 82C. There will be small performance differences (< 5%) and it must be said that premature electronics failures can be temp related. A certain but extremely small % will fail within the warranty and the % that fail at 75C will be a hair higher than those that run at 70C.
So now to your question ...
1. Two hybrids in a system is a logistical issue so I would throw that one right at the window. Low efficiency aluminum rads need hi rpm fans which is noisy option.
2. An air cooled system will be just fine, tho the top card will fun hotter as it's using the bottom card's hot exhaust as intake air. You can alleviate this with 1 500 rpm 120mm fan mounted on back of HD cages... many cases (i.e Phanteks come with predrilleld holes just for this purpose).
3. Of course a custom loop is the best solution as it will be dead silent if properly sized.
4. One option you may not have considered but if you find a custom loop too much of a leap, an Open loop cooling system would seem ideal for your goals.
A. Start off with a "open loop", quality all-in-one unit like the Swiftech h320 X2
http://www.swiftech.com/h320x2.aspx
This will easily cool your CPU and 1st GFX card. The unit is pre-assembled at the factory using custom loop components which are all available separately. Reviewers report it being easier to install than popular CLCs and it destroys them in cooling and lower noise.
B. Purchase a GFX card with a
full cover water block which water cools not only the GPU but also the VRMs and memory. The WB is made by EK which is the best performing unit on the market.
https://www.newegg.com/Product/Product.aspx?Item=N82E16814127956
You will need to buy 2 fittings, some extra tubing and coolant....instructional videos can be found on youtube
C. When ready, buy your 2nd GFX ... and if desired, add a 2nd radiator for extra cooling.
Now all that being said ... understand that if you compare these 3 systems:
a) SLI system w/ custom loop
b) SLI system with Swiftech AIO and MSI Seahawks
c) Air cooled SLI system
I can pretty much guarantee that you will not be able to detect any difference in performance....it might be there as the better built componentry may deliver single digit % better benchmark numbers, but tho the better components **could** deliver more fps, they are nerfed by Boost 3 which doesn't allow more overclocking no matter what the temps are. Its strictly a matter of "is the card throttling or is it not" ... and that's not going to happen unless GPU temps break 82C or your VRM / memory gets too high . The only cards that do get those high temps are the FE models, ones with FE type blower coolers and the original EVGA SC and FTW cards.
Remember tho, heat is not the only reason to water cool .. if done properly, they are much quieter and more aesthetically pleasing.
NOTE: If you do the SLI thing water cooled, I'd advise running the 2 cards in parallel ... that is split the flow before the cards into two tubes running one to each card than back together after the cards so as to send ha;f the flow thru each card.