The basic idea after I tried on the Pixel 2XL:

you unlock device as usual (enable developer mode, authorize your PC for USB debugging, boot into bootloader do fastboot flashing unlock) Then, do not try to install TWRP permanently. You will have to boot it everytime with fastboot boot twrp-xy.img You can select the slot to flash to also with the fastboot command. What I did was: Boot TWRP in slot a Install LineageOS 16.0 (to get the vendor partition), TWRP will install it to slot b! Flash halium-boot.img to slot b Install rest of system with halium-install. You also need to specify flag -s to install it with "system-as-root" mode

System is booting now, but recovery cannot be installed ATM with Halium. We are doing work now to integrate our recovery properly, at least then you will be able to use it.