I've got good news for you...and I've got bad news for you. Since my wife always prefers the bad news first, so as to end on a good note, the Corsair One (ALL configurations) only comes with one graphics card, and will ALWAYS be able to hold only one graphics card. This is NOT a bad thing, as I will explain. But, first, the good news...not only can you upgrade the RAM, but doing so no longer violates the warranty. Originally, any internal "tinkering" would, by Corsair's decision, would have violated the warranty, at least according to Corsair (courts of law, on the other hand, would have countered that, as Corsair's warranty cannot be canceled for such reasons, according to US law). However (and possibly because of US law), Corsair recently changed their T&C to indicate that owners can change certain components without violating the warranty, including the RAM, SSD, and HDD (for models equipped with a HDD). As such, I've replaced the Corsair SSD with a Samsung SSD, and I've upgraded the original 16GB (2x 8GB) Corsair Vengeance 2400MHz RAM with 32GB (2x 16GB) Corsair Vengeance 3000MHz RAM.
Getting back to my "NOT a bad thing" comment...I've been custom-building desktop PCs for over 20 years, and, in all that time, I have never seen any previous pre-built systems worthy of truly being considered "hard-core gaming systems). Most were under-powered, and for those having the hardware, they were overpriced. Looking at the "boutique" companies, they (quite obviously) had the hardware, as well as no 'bloatware', but they have always been way overpriced. You could easily build yourself the same system for a fraction of the cost, which no "bog-box" company could compare/compete with...until now. Boutique companies have always had to purchase from vendors (with some components coming from the manufacturers), thus they're paying profits for 1-3 other businesses. Then they mark up their cost when selling to customers, so as to make a considerable profit, themselves. Corsair, on the other hand, IS the manufacturer of the majority of the components, thus they are not paying profits to other companies. The only exception would be where the mobo & GPU are concerned, for which they've struck an incredible (and "interesting") deal with MSI.
While the GPU is the exact same MSI graphics card you could purchase in a retail store, the mobo is a semi-custom design, manufactured specifically for Corsair, which cannot be purchased in retail. As for everything else, since Corsair IS the manufacturer, their costs are greatly lowered. I've priced out all the components, based on street prices (except for the mobo & case, both of which I had to estimate), and what I determined was that the price the various trims of the Corsair One are available in, your cost to build something equal would cost you approximately the same. Additionally, there is NO 'bloatware' installed, and the ONLY additional software installed is Corsair's own CUE (Corsair Utility Engine), which you can easily uninstall (I've left it installed, as I have a Corsair Mechanical keyboard, & Crsair headset). I use the mobo's "Game Boost" setting, mildly OCing the CPU, plus I've OC'd (by a considerable amount) the GPU. Having benchmarked my system, it is on par with high-end boutique-made systems. Put another way, you will NOT find a better-made pre-built gaming computer for this price.
If you have the time to build your own gaming computer, by all means, go for it. Building is fun...plus you get the satisfaction of knowing you built it. On the other hand, if you don't know how to build a desktop and/or you just don't have the time, then the Corsair One (with the exception of the base model) is your best bet, your best "bang for the buck", and the most powerful pre-built gaming system you will find, all rolled into one. as a quick "add-on" note, there are still quite a few games which can't utilize multi-GPU systems, thus having a dual-GPU (or greater) system can be a waste of money. Best of all, the Corsair One is extremely quiet, and runs very cool. You can't say that about most boutique-built gaming systems...they might be quiet, or cool-running, but (almost) never both. Best of all, if you're interested in VR, you'll also have a system that not only meets, but greatly exceeds, the "recommended" components (the Oculus Rift runs absolutely fantastic on my Corsair One Pro).