Lutfij :
two cards will give me twice the power
In what? Gaming? Nvidia have made it abundantly clear that SLI support will have to come from the developers. If the game you want to plat or are playing does not have support for SLI then you're just burning through your utility bills and asking more power from you system. The ideal path is to buy the single most powerful GPU your budget will allow.
In terms of cooling, it also depends on what you want to do. You see custom watercooling is more of a hobby rather than what is deemed necessary. So much so that it's used on showcase builds merely due to aesthetics. Yes you do get temperature benefits when you have the GPU under water but with advancements in air cooling and the lithography of chips you can get away with air cooling.
Mind you, custom level watercooling isn't cheap. You could pass on your full system's specs + in the mean time read through the watercooling sticky(linked in my sig space) to get a better grasp on the world of watercooling.
I forgot that part for a second yes, support for sli is not something that is incredibly avalaible right now, but isn't that coming up in the future ( im also looking for a bit of a futureproof build which is hard to get in such a developing sector as pc hardware)
Here are the specs i currently have in mind:
CASE: CORSAIR CRYSTAL 570X RGB
CPU: AMD RYZEN 7 1800X / 4,00GHZ
RAM: G.SKILL TRIDENT Z RGB DDR4 3000MHZ 32GB
GPU: GTX 1080 TI 11GB (still waiting on a good brand release)
MOBO:ASUS CROSSHAIR VI HERO
SSD: SAMSUNG 850 EVO 1 TB 2X
HDD:SEAGATE BARRACUDA 1 TB
Power supply: 650W CORSAIR RM650X
Windows 10 home edition
I've already had quite the research trouble with the cpu but in the end i picked the ryzen over the 6900k due to it being newer, although not entirely stronger but alot cheaper for that bit of performance it loses.
I've already calculated how much a custom loop is gonna cost me and it'll be a bit more than a second gtx 1080 ti, that's why im now in this dilemma