RE: Dell M.2 / NVME Specifications And Upgrade Requirements For Latitude, XPS, Precision Mobile and Optiplex
Thank you for your questions.
The 5580 can't have one 2280 M.2 and one 2.5 inch drive installed at the same time. It can have one 2242 drive and one 2.5 inch drive at the same time. The 2242 goes in the WWAN slot but can only be used a cache drive.
Your choices are one M.2 2280 as boot or one 2.5 inch drive as boot. A battery downgrade to 4 cell, does allow for a 2.5 inch drive to be installed, if you so choose to switch.
And you are correct. 5580 with U processor i5-6300 supports 2X NVME.
Parts required to switch to 2.5 inch:
1x 6F7DD HDD bracket
1x 6NVFT HDD cable
4x 4270E screw
Dell Technical Support L3 | Project Sputnik
Dell Support is also on Twitter! https://twitter.com/DellCaresPRO