Hello, old buddy, how're keeping up?
If I remember correctly, you're on msdos/ bios-legacy setup.
if that is the case, and only in this case, install KaOS as you would any other linux installation (remember boot up install media in bios-legacy). Then install without installing the bootloader (if you muddle this up, don't worry, we'll get this back for you at kaOS).
Then at your Endeavour or Manjaro OS, do a simple update-grub (or grub-mkconfig) to include KaOS in the bootloader.
Good to hear from you. Say hello to my old friends, particularly that spunky Aussie girl, Kadeo.
ps: if you are now on uefi/gpt, set aside another EFI partition and remember to mount this as KaOS /boot (not /boot/efi). KaOS will install its bootloader as systemd-boot not grub, then boot up (selecting from computer bios setup) your Endeavour (or manjaro) and do an update-grub) from that OS.
ps; I personally have my own way (I have systemd-boot, grub, refind and uefi and bios-legacy all in the same system)
so I don't do the way I ask you to do. But I think you know that. That that should work. I'm sure - unless.... you muck up 🙂