Do you already have one or both? Why do you want to have a separate one for OS, and another for apps? As said above, the 2nd slot will run at ~half speed, better off just getting a single drive that's big enough for whatever you want to put on there. Most use cases don't really benefit noticeably from an NVMe drive compared to a SATA one anyway.(Samsung 970 Pro ) i want to use one for apps and games one for OS ???