Maybe it doesn’t like the new Kernel driver. You could try…
osmc@RASPBERRYPI:/boot$ sudo mount -t exfat-fuse /dev/NASvg/NASlv /mnt/NAS
Although I’m not sure what the point of combining these drives would be. It seems like it would be simpler to just add both drives in Kodi inside the same source location which makes Kodi display them combined as if they where one browsing through ‘videos’. Or you could just use a library in which the actual locations are irrelevant.