Dual boot Manjaro and Debian?

[dieselnutjob]

Hi
I have just installed a 128Gb eMMC in my PBP, and now I would like to install both Manjaro and Debian (with 32 bit userspace) onto the same eMMC with a boot menu.
I think that I already know how to write a boot menu in uboot.
What I don't understand is how kernels will work.
More specifically both Debian and Manjaro use an "Image" file for the kernel.
I presume that the "Image" file is under package management control, and so I can't have two "Image" files in the same boot partition which are controlled by two different package managers.
Also, I do know from messing with other ARM64 systems that it is possible to have a Debian userland with no kernel package installed at all, so not in control of the Debian package manager.
Could I use the Manjaro kernel, under control of the Manjaro package manager to also boot Debian 32 bit userland?
If so then that would be a good solution.
Thanks.

[dieselnutjob]

I managed to install and boot Debian in a separate partition.
The steps were:-
Boot Armbian on microSD.
Use gparted to shrink the Manjaro partition.
Create and mount a new partition.
Use debootstrap to install Debian into the the new partition.
Chroot into the new partition and set the root password.
Copy the Armbian kernel and dtb files into the boot partition but with different file names so that they don't clash with the Manjaro ones.
Configure extlinux.conf with a menu and parameters to boot both either Manjaro or Debian.

So far it works, except that I cannot get x windows to display in the Debian partition.

[dieselnutjob]

This morning I found that I can actually boot the default Manjaro kernel with Debian 32 bit root filesystem, so one single kernel for both.
The problem is that I can't see the console.
As the kernel boots I see a few lines and then the screen flashes (I guess it is starting framebuffer console?) and I can no longer see anyway.
x windows is already disabled (with 'systemctl disable sddm') so it's not that.
At one point I was able to blindly type in root, the root password and then reboot, and it did reboot. So it is alive, I just can't see anything.
I added 'nomodeset' to the boot options but that doesn't seem to make any difference.

Is there a definitive boot option that I can put in /boot/extlinux/extlinux.conf which will stop the kernel using framebuffer?

---

hmm maybe I'm wrong. I set up a serial console using my FTDI serial null modem. Works with the Armbian kernel but not the Manjaro one.

[dieselnutjob]

For the time being I just used dd to copy the rootfs partition from my Armbian microSD card over a partition on the eMMC.
It works.