According to system storage specifications, there are both PCIe gen4 and gen3 being used for M.2 slots.  That was the limitation if one of your drive is on gen3 slot.  Check the transfer mode on each slot to know for certain.  It's harder for utility apps to detect slot in RAID volume.  You may also swap the boot drive to test each slot. 
I have a new Precision 3660 Tower, this came with an i9-13900K and a basic 256gb SSD I've installed installed an RTX A6000 and 2x Sabrent Rocket 4 Plus 8TB m.2 drives, using the Intel Storage Management app I'm using both 8TB drives in a RAID 0, I'm only getting read/write speeds which are just higher than a single drive, is this a limitation on the m.2 slots? Single drive read/write is up to 7,100/6,000 MB/s RAID 0 read/write is 7,087/6329 MB/s
Precision 3660 RAID 0