Hardware M.2 SSD raid only possible with Apple Embedded controller such as the one that the iMac Pro uses but these are soldered on to the motherboard and use a Apple proprietary version of the M.2 interface specification.
The only way you'll get two standard M.2 SSD's working in a Raid is to use Apple's Software Raid. Whilst the Apple's Software Raid is good its not perfect and it can and does crash causing data loss. Its rare but it does happen.
You would get a much more stable and secure system by using one 1TB Samsung EVO 960, the read and write speeds are fantastic (I have one in both my laptop and desktop systems) and more than fast enough to edit 4K video with no read/write lag.
If the 960 Pro is not fast enough for you You could try the new
Samsung 970 Pro M.2 SSD's which has around 30% faster read and write speeds compared to the 960 Pro's.
Cheers
Jay