Q. Can I put two mSATA SSD’s in my Dell Latitude 6430u?

No, you can’t.
Yes, there is a spare Mini-PCIx slot for a WWAN card. Yes the mSATA SSD card will fit in that slot.
Unfortunately neither the BIOS or the OS will see the storage.

Here is why – http://en.wikipedia.org/wiki/Serial_ATA#mSATA