Hi @Clovis 🙂
As a current user of Xiaomi Notebook Pro who has added a second SSD, I can tell you that this will work just fine.
The primary slot is PCIE x4 (bandwidth 3.94GB/s) so the 960 EVO will work just fine and with full speed, the secondary slot is 2x (bandwidth 1.97GB/s) so the original SSD might be restricted a little bit but I’m sure that it won’t really be noticeable. Both slots support NVME and SATA M.2 drives.
You can easily find a lot of tutorials on how to open the laptop, for example:
It looks fairly simple but I must admit that I’ve failed to do it myself, as I didn’t have the proper tools or skills. In the end a computer workshop has done it for me for a fair price 🙂
Best regards,
Maciej