Now, I admit I'm speculating on the "dual drives support" thing, but it seems kinda obvious XSX and PS5 are going to be utilizing a setup very similar (and improved upon) from AMD's SSG line. Just...nowhere near as expensive (mainly because of much less memory and using non-SLC or MLC NAND).
I think the whole "dual drive" theory is very plausible. Both Sony and MS need to know the speed of the drive they are using as virtual ram, otherwise it'll ruin a lot of the optimizations the devs are making. I suspect it could be 256GB (or maybe 512GB) of ultra fast storage, and then a 'standard' NVME SSD which can be replaced... well, not officially replace and will void warranty, but still replaceable.