I have a Latitude E7270 and can confirm that the WWAN/HCA slot CAN accept a 2nd SSD that is 2242 size (22mmx42mm) and the Western Digital SN520 series drives work.

Disclaimer:  Basically, the 2242 PCIe NVMe M.2 2242 drive needs to have a physical B-slot and M-Slot such as the Western Digital SN520 series.  The typical PCIe NVMe M.2 2242 format cards only have an M-slot (such as the Samsung PM991 MZ-ALQ1280) which will NOT fit as the 2nd SSD port is physically set up for B-slot cards.  Only a few manufactures make a PCI-e NVMe M.2 2242 SSD with both the B-slot and M-slot, and the Western Digital SN520 series is one of those.  NOTE:  A SATA based M.2 2242 SSD will NOT work...the M.2 2242 drive must be PCIe NVMe. 

So if you are looking to add a 2nd drive to your E7270, it is possible, and the Western Digital SN520 series  2242 works.  I tested it with both a Western Digital 128GB and 256GB SN520 Series NVMe 2242 sized SSD, model numbers SDAPMUW-128G and SDAPMUW-256G.  The E7270 recognizes the drive with no modifications to any BIOS settings.  

Hope this helps.