Pretty much as the subject says. I'm looking for an Inspiron 17" laptop that can have two drives.
The current 5000 and 7000 level Inspiron and G7 models will take an M.2 NVMe or SATA plus a 2.5" drive. If you're looking for a current Inspiron that will take two 2.5" drives - there aren't any. If you're looking for a system that can take two M.2 drives -- among the Inspirons, there aren't any. That feature comes at the Alienware or Precision mobile workstation level.
Looks like the Inspiron 5770 can have a NVME but not the 5767. Do you know if the NVME bus on the 5770 allows for the maximum speeds?
Yes, I have two SSDs in my Inspiron 17 5767. The connection is SATA III. There is no M.2 port. You have to replace the boot drive with a SATA SSD and use a media bay caddy for the second SSD. The optical drive occupies the media bay and you simply purchase the caddy online to replace it. You should also make sure you have an external DVD drive to replace the media bay drive. Use F12 during power on to boot from media in the external DVD drive should you need to reinstall Windows.
If you are hoping to have both an internal 2.5" SSD and an internal M.2 SSD, you do not have the necessary M.2 port to do it. There are two kinds of M.2 ports in some modern laptops: One is keyed for an SSD and the other is keyed for WiFi cards. They are not interchangeable so consult the documentation for which is which in any system you are considering.