Both the top M.2_1 and bottom M.2_2 sockets can operate in either SATA or PCIe mode. Enable PCIeX4 mode for the The top socket where your 960pro NVMe drive is and use the bottom socket for your M.2 SATA SSD just leave the setting for M.2_2 as is, it defaults to SATA when inserting a SATA M.2 SSD. You will be fine. The 960Pro will use PCIe lanes at X4 and the 860evo M.2 will use SATA.
The PCIeX options present for M.2_2 are for when one wants to use a second NVMe drive in M.2_2 and wants to enable PCIeX mode. In that scenario one might run into bottle necking though, as the M.2 sockets both are getting and utilizing their PCIe lanes from the chipset.
Though honestly IMO, it's more cost effective to use a 2.5" SATA SSD for additional storage unless you are out of mounting options or (and I can appreciate this) just don't want to run any more cables for power and data to keep things clean and tidy looking.
Other systems in addition to rig in the profile specs:
Aorus Master x570, 3950X, 32GB GSkill c16 @3800, Sabrent PCIe4 1TB, Sabrent 1TB, 860Evo 1TB, 2080 Super, HX1200i, Celsius S36, Define S2, pg348q
Aorus Master x570, 3900X, 32GB GSkill c16 @3800, Sabrent PCIe4 1TB, 860Evo 1TB, 5700XT, HX1000i, H150i Pro, Obsidian 500D, LG32GK650F-B
MEG x570 Unify, 3800X, 32GB GSkill c16 @3800, Sabrent PCIe4 1TB, 860Evo 1TB, GTX1080Ti, SSR-850PD, Celsius S36, Define R6, u3415w