Hi,
I managed to dual boot Android 11and Ubuntu touch, but I did not use ubport installer as I was not sure what it does exactly to the memory layout of the device.
my device is Oneplus 6t with a dual boot recovery.
using A/B/C partition layout.
32GB for Android 11 and 32GB for testing other OSs and 64GB for shared Storage.
I used this guide to install Ubuntu Touch, after I installed Android 9 on testing partition.
so far it works.
my question is about UBport installer, does it install UBT to the active slot or does it wipe everything ?
I'm too busy right now to browse the source code to see what it does.
Thanks

@nero355 with a Custom Rom, It works. I tested many combinations of Operating systems and Android versions.
currently I'm on Android 11 (Evolution X 5.9.2 Rom) on Slot A and Ubuntu Touch on Slot B.
I want to try the installation with ubports-installer, but i'm not sure if it flashes to the active slot on boot-loader or does it change both Slots.
I cant risk losing Slot A, I have many mods on it, even Nand backup won't restore it to the current state.

Thank you all, I went through the source code and the installer log.
All it does is flash boot.img and vbmeta.img to the current slot and does not touch the other. the recovery image does not work so I fastboot boot a working recovery and continue from adb shell.
Then it downloads the xz.tar files with their keys and push them to the device /cache/recovery once it's in recovery mode, then it adds recovery.fstab to /etc/

The downloaded files come with ubuntu_command file that is executed in recovery using system-image-upgrader script.
the script formats system_b partition and load keyrings then extracts the xz.tar files in cache, and strangely tries to move cache/data/* to /data when the first one doesn't exist, because the extraction results in system folder not data. (I guess I will change the path)
Anyway now that I'm sure the main slotA does not get touched I will keep testing until I come up with something that works. (I already got it to the boot splash but it keep loading and does not go past it)