Are we dealing with M.2 NVME drives, and, hopefully both mainboard slots are NVME-capable M.2 slots?
As long as both slots are enabled (some mainboards disable SATA ports when M.2 NVME slots are filled), and if both are NVME X4 lanes, it won't/should not matter.
If dealing with NVME and SATA M.2 capable slots and drives, you'd want your OS on the NVME drive...
If this is insufficient answer, a few more details (or, any details, really) on your mainboard will allow someone to assist further by researching the mainboards/specs/manual...