That's because you don't read instructions:Did everything exactly as you said on the actual oneplus 8 pro thread this time. Same exact thing only when i install it it actually installs no problems only to reboot after factory reset into fastboot bootloop mode. seriously dont know whats wrong and i have no idea how to put android 11 in both slots prety vague about that one if u ask me.
you need to have stock Android 11 in both slots. So, either update stock OTA to populate the second slot or flash stock rom with TWRP.
Read 8 pro general section: you will find a thread for all stock roms. There are several methods: you can use MSM to OOS 11 and then use TWRP to flash to the second slot. Or you can use TWRP to flash once, reboot, then back to TWRP and flash again. TWRP could be booted via fastboot or installed.