I've closely similar machine (Precision 3450) which uses same printed motherboard layout but with slightly different components on it (e.g. quality, added connectors {system fan}, etc.). Some differences are: PCH is W580 instead of Q570, extra M.2. slot (CPU bound, PCIe Gen4).All of my storage ports are connected ( two of Gen3 M.2, one Gen4 M.2, three SATA {two SATA SSD drives and one DVD} ) and operating within target specifications (transfer bandwidth).Nvme M.2 Gen4 - 7 GB/s (CPU bound, x4 lanes, another x16 reserved for GPU to CPU link, blue slot, total x20)Nvme M.2 Gen3 {a} - 3.6 GB/s (x4 PCIe lanes)Nvme M.2 Gen3 {b} - 3.6 GB/s (x4 PCIe lanes)0) SATA2 {a} - 150 MB/s {DVD} (x1 PCIe lane?)1) SATA3 {b} - 570 MB/s {M.2 SATA drive via adapter} (x1 PCIe lane?)3) SATA3 {c} - 570 MB/s {SSD SATA} (x1 PCIe lane?)One (second) SATA port has no connectivity implementation from chipset.Even have native Dell SD card reader installed (blue port on front of MB, Realtek PCIe Gen2 x1 lane implementation)Both PCH/Chipsets (W580/Q570) have exactly same DMI 3.0 to CPU link (7.8 GB/s), chip pinout, PCIe lanes support (up to x24 gen3) and SATA controllers with RAID (but max # of SATA 6.0 Gb/s Ports is 8 vs 6)Therefore technically there are no constraints from storage perspective to use those interchangeably, if there is any limitation for concurrent use of SATA{RST}/Nvme lanes then it's rather artificial most likely implemented on BIOS level (with probably some missing circuit board elements).I was about to buy Optiplex 7090 SFF too (because Precision 3450 SFF is almost the same) but last moment realized it doesn't support Gen4 Nvme storage (missing connector right above PCIe x4 slot towards CPU/VR). [IMAGE]    
Based off the PDF [URL] it seem Dell is making the claim in Table 14 (Storage Matrix) you can't have dual NVMe PCI M.2 with dual SATA. From Table 10 the two 2280 M.2 slots are PCIx4 NVMe slots so they shouldn't consume a SATA port. So it seems that dual M.2 2280 and dual SATA SSD would work?  Is there an artificial limitation created by Dell that makes this unsupported?  I'm less concerned about Dell tested configurations and wonder what is technically possible. 
Optiplex 7090 SSF dual M.2 with caddy dual SATA SSD