Neat, but I see it personally as the worst of both worlds, unless you have a bunch of NVMEs sitting around.
You're going to be bottlenecked by SATA speeds, so even one NVME would be bottlenecked, let alone 2. So for me, going with a larger SATA SSD (which you could of course RAID with another) would probably get you still better speeds.
Then you have issue of it breaking. Personally, I have never had good luck with secondary board RAID items like this. They always fail after a while. The only stable raids I have seen are motherboards and SAS. Whenever I see "Make this interface into another RAID" I think of the.... 5-7 failed cards sitting behind me.