Yes, I have two Corsair MP600s on it. But I wouldn't save money here, but rather use two PCI-E 4.0-compatible SSDs. Either way, you need something fast for the OS and games (e.g. Flight Sim) benefit significantly from a fast M.2 SSD.

Yes, that's right, you can put 2 M.2 SSDs in there. In general, if you want a "small, fast" and a "big, slower" SSD, just buy an M.2 for the "small, fast" and a SATA SSD for the "big, slow", you'll hardly notice the difference and your wallet will be happy too :)