Identify the operating system in the BIOS settings.
Check Asus Bios settings to identify which SSD is the primary drive and which is secondary. Adjust the boot priority for the desired SSD.
Observe the OS within Disk Management. Simply identify the c: drive.
He stated his perspective clearly.
In BIOS you can't access this. Try removing one and check if it starts. This will help identify which port hosts the operating system.