Not to offend ANYBODY and it's just my opinion, you minds well toss your money in a slot machine for all the good they do. And you want to open it up to 2 GPU'S?
So the blocks for the GPU'S will be a waste and useless after(only good for those cards), IMHO a total waste of time and money. Your never gonna see good enough temps on the GPU'S to even go much higher then the normal overclock on the stock cooler. For $100+ on each GPU? plus all the fittings, if your gonna water cool use a pump, RADS,REZ,BLOCKS, tubing and fittings are up to you. Plus if you go the custom route everything but the GPU blocks is re useable...........Money well spent.
(HINT: BITS POWER FITTINGS JUNK!)
I finally just found the OC'ing sweet-spot for my two MSI GTX-970s. While you should be able to find a stable 1500 Mhz for the core of a single 970 and even 8,000 Mhz for the memory, two in SLI may change the equation. I find myself bumping up the voltage 15 mV to secure a clock of 1,470, and I'm only pushing the memory to 7,500. This keeps the air-cooled temperature on the upper and hotter card to 76 or 77C depending on room ambient between 78 and 80F. That is, the temperatures occur under FurMark 4xAA 1080p stress-test loading, Heaven with similar settings doesn't exceed 65C. The maximum range on the cores of these cards is about 1600, but the TDP or power limit can't be loosened beyond 110%. Since I'd like to keep the maximum temperatures below 78/80C, and since gaming loads under this regime are closer to 55-60C range, Why would I spend $122 each for EK full-cover waterblocks, and all the other parts -- reservoir, D5 pumps, fittings, etc.?
On the other hand, if one invested in the latest 12GB Titan cards, they'll be good for so long, and you'd be advised to sell them sooner to keep up with the next iteration, or keep them as long as you can. If you invest in a single $1,000 Titan card, you might think it wise to spend $122 on the waterblock. And obviously, if you invested in two of them, you'd double the waterblock expense to $244.
So it's the difference between investing $700 for two 970 cards, $1,100 to $1,200 for two 980 cards, and a couple Grand for the El Supremo Titan cards. It will affect your preference for cooling. At the lower end of total outlays, you may do just fine for air-cooling.