XPS 15 9510 second SSD options

I'm considering the XPS 15 9510. Regarding SSDs, the Storage section in the users' guide says this computer supports the following storage options:
● One M.2 2230 solid-state drive (up to 256 GB)
● One M.2 2280 solid-state drive (up to 4 TB)

Ideally I would like to buy a system with a 1 TB SSD and possibly add a second 1 TB internal SSD later. But if I'm interpreting the specs correctly, that wouldn't be possible. The initial drive would occupy the 2280 slot, so the second drive could be no larger than a 256 GB 2230. Am I reading that correctly? Has anyone successfully configured this model with two SSDs larger than 256 GB?