Its amazing what you can find with a simple google search (this should help you) :

Just like what they said with a tiny bit more clarification.

Your NAND is stored in a little Hynix/Samsung chip on the board. (Not sure if MS utilizes more than those two brands)

With Dual-Nand capabilities, you essentially get to have a JTAG/RGH and a retail console all in one.

You install your JTAG or RGH NAND onto one of them, be in the NAND chip stored on the DemoN or the one stored on the motherboard itself. The one that you don't use for the JTAG/RGH Nand is the one that gets the Retail NAND.

By doing this, you can then use a switch (Not sure what the DemoN is going to utilize...from TX, I wouldn't be surprised if you can think about it..and it switches to the nand you want) and select which NAND you want to play from. If you are wanting to utilize homebrew, switch to your JTAG/RGH Nand and play OFFLINE. (It is always recommended to put a different KV onto your JTAG/RGH Nand so you don't run the risk of compromising your Retail KV, although it is not necessary) When you get tired of playing on that one, you can flip the switch and power cycle the xbox and then connect to XBL and play retail games, etc. like a normal unmodified console does.

You can boot into the unmodified kernel (retail like it is now and use XBL), or flip the switch and boot into the Modified RGH kernel (play homebrew ect ect) and can switch back freely to whichever you desire. Yes it is a RGH/Retail all in one system by utilizing a separate chip that stores your spare nand.