You have EU or Global? And mind sharingYes you can update the Firmware from the Firmware thread and that will get flashed to both slots, but I think there it is for Android 12 the C11 one, but there is already a C21 one existing now but not uploaded as flashable firmware yet.
I Have build my own flashable Firmware file for the A12 C20 Firmware for my Oneplus 8t but have not flashed it yet as I am still on OOS11 Base right now.
Will switch to OOS12 Base soon as the next Update for my custom ROM will need that.
Thnx in advance