Can the Dell XPS 15 9550 w/ 4K Touch Display be configured with both a PCIe SSD and a 2.5" drive? Dell Chat Support says "yes", but I have also read that the 4K Touch Display requires the 84WHr battery which extends to fill the space of the 2.5" drive. At 4:44, in the YouTube video below, I think we see that there is no room for a 2.5" drive. But, maybe Dell has a different model option that supports 4K Touch and dual HDD. This question has probably been posted before, but I didn't find by searching.
If you want the UHD screen, the larger 84 W-Hr battery is mandatory - you do indeed lose the 2.5" drive bay. The only way to increase internal storage in this model is with a larger capacity M.2 PCIe/NVMe drive.