I have a XPS15 9550 laptop (with the 1TB SSD option) The SSD is installed in the conventional storage bay. I ran the intel rapid storage app and it implies there is another SSD bay inside, but currently empty.
Here's a screen grab. I think the empty internal port is for a mini SSD 'stick' type hard drive. I think it's an "M.2" type slot but I'm not sure. Would one of these fit?https://www.amazon.co.uk/s/ref=nb_sb_ss_i_5_3?url=search-alias%3Daps&field-keywords=m.2+ssd+1tb&spre...
I know this is an older post, and while this is somewhat out of scope from what was configured with the system originally an 80mm (2280 form factor) drive that uses uses M-keying will fit. You will want to make sure that there is not anything already in place for this slot and can do so by following the instructions at this link: http://dell.to/2ef5D8S I hope this helps. If you do a search online you will see various detailed posts/videos explaining the procedure as well as drives that have been used.