First of all, the standard NUC 4x4 form factor does not leave enough real estate to fit (and properly cool) a second 2280 M.2 drive (regardless of whether NVMe or SATA). Skull Canyon was a larger form factor board and had the extra room for the drive.
Second, the Skull Canyon design utilized a Mobile (as opposed to Embedded) processor, with a separate chipset (PCH), and this provided the additional PCIe lanes that would allow a (second) NVMe SSD to be supported. The Embedded processors, which are System On A Chip (SOC) designs, have an embedded (barebones) PCH and the PCIe lanes needed to support a second NVMe SSD are simply not available.
The NUC team is working on the next-generation designs - and this does include another "extreme" design - but details are not yet being made public. I can assure you that they understand the value proposition for multiple NVMe SSDs - and that this is being considered - but no commitments can be made at this time...