1) Yes, running 2x 1TB Intel 660p NVMe drives in PCI-E 3.0 x4 RAID-0 on a B450
2) Cost of storage, a 2TB drive at the time was more expensive, I got 2x 1TB NVMe SSDs, for cheap.
3) Some performance increase. The 660p is spec'ed at 1800/1800, I get 3000/3200 seq. read/write. (A fast single NVMe drive, like an HP EX920 / EX950, or a Sabrent Rocket NVMe, or a Samsung EVO 970 / EVO Plus 970 can outrun that.)
4) Sure, if your board is capable. My board may be nearly the only B450 board that supports DUAL PCI-E 3.0 x4 NVMe slots. Most B450 boards, the secondary NVMe is PCI-E 2.0 through the chipset. Mine is bifurcated off of the GPU's PCI-E 3.0 x16 slot, I think.