Hi rong1966,
I could not find out how to successfully hook up a CGA 347 valve to a NXG2 valve (because of the quick release nipple).
I decided to chuck my NXG2 valve all together, It had problems anyway, as it was old and the knob for air flow did not turn smoothly.
I've opted for a new valve entirely.
The Scott Carbon air Tank I got has 7/8-14 UNF threads. A new valve (on Ebay) can be had for $55 to $75 (sold from inventory here in the U.S. and China)
Amazon has a valves for sale, too! both. I already own a 3 liter tank with a M18 1.5 -6H valve. I love how smooth it works.
My Scott tank needs a 7/8-14 UNF threaded valve (On Amazon, they sell for $85 to $107)
I ordered a 7/8 - 14UNF valve on Ebay. When I got the valve, it screwed on perfectly snug to the Scott tank. I was optimistic. The Problems began (with the new valve) when I hooked it up to my Yong Heng Air Compressor to fill my tank. The valve proved defective. While air was being pumped in, the gauge needle wasn't registering PSI----so I returned it.
That's where I stand today. Even though I have a tank---I'm Still valve-less.
If I were a "well to do" air gun enthusiast, I would have ordered a brand new DOT 9L tank with all the fittings. I am what you would call a budget minded air gunner. I truly love air guns, both the language and technology. I enjoy the zen focus of target shooting. I own 7 air guns and I'm always looking for more. It's a Terrible (I smile) addiction. Having been around, I know of worse vices---But---This one ( I smile again) is tame in comparison.
Another alternative for your Scott Tank is to get a "7/8x14 Male to M18x1.5 Female Adapter For Carbon Fiber Cylinder Conversion" on Ebay. they cost between $15 to $20. That way you have the option using either sized valve on your Scott tank.
Good luck, my friend.