XPS 15 9500 Storage

I am unsure about the description in my Setup & Specs document.  It states "your XPS 15 9500 supports one of the following" then lists M.2 2230 & M.2 2280.  Since the device has two slots for M.2 devices, then  does this mean that I cannot add a second M.2 device?  Or does it mean that my XPS can have two M.2 drives provided they are both M.2. 2230 or they are both M.2 2280?

Can anyone please help me to resolve this dilemma?