Dual booting Ubuntu 24.04 and 25.10

[Gort]

I want to dual boot Ubuntu 24.04 and 25.10. I have Ubuntu 24.04 install and use grub customizer. In what order should I install Ubuntu 24.04 and 25.10 so that grub customizer from Ubuntu 24.04 be the primary boot order.

Gort

[oldfred]

I am not a fan of grub customizer, it replaces standard grub scripts in /etc/grub.d with its own proxy versions.

Not that difficult to add your own boot stanza into /etc/grub.d/40_custom. When a beginner I just copied stanza from one install into the other.

Last install is the one in control. Both installs must be in same boot mode, and most systems are now UEFI, so both must be UEFI. If on different drives, still better to add an ESP - efi system partition (FAT32) on both drives, even when only using one as main boot stanza.

Major updates to grub can reinstall grub making it the default. So then boot into your main working install and run this to make it default:
sudo grub-install

I prefer to label partitions, so use label. Then 40_custom entry can be just like my example with is plucky on my sda drive so label is plucky-a, I add simiar stanza to my plucky-a install to boot main working install & turn off os-prober. Updates are a lot quicker as it has to probe many partitions looking for an install.

menuentry "Ubuntu 25.04 plucky /dev/sda8" {
    search --set=root --label plucky-a
    configfile /boot/grub/grub.cfg
}

[Gort]

Your reply is very interesting and informative. It is greatly appreciated. Thank you so much for your insight and wisdom.

Lastly, how do I mark tis thread as solved Discourse is new to me and I also would like to show this post with accolades to the forum.

oldfred I have read and applied all answers you gave to me. Many thanks to you and all who have made me better Using Ubuntu, I also add these post to my Tutorial folder on my favorites bar so as not to ask the same question again

Gort

[rubi1200]

You can use the checkbox in the relevant post to mark as the solution.

As already mentioned, Grub Customizer is problematic, to say the least.

Although it looks intimidating at first, creating custom GRUB entries is not that difficult once you know the correct syntax to use.

[corradoventu]

I have multiple Ubuntu versions and Debian too and don’t use grub customizer, grub is great at doing it yourself

[Gort]

Yes that is true corradoventu. I have used it for a long time withe no problems. I do have a link saved in my Ubuntu folder, a tutorial one editing grub, I study it now and then until I can grasp it more effectively.

Gort

[guiverc]

I do what others have said.

My system is dual boot; and when I need to re-install an OS; I just do so, and know post-install that just installed OS will control the boot process (ie. it’s GRUB will be the one used).

When I know the system is working as I want, I just boot the OS I want to control grub, and re-run its grub-install so it takes over ownership of boot.

I do that on my current uEFI box (which contains Ubuntu 24.04 LTS & Ubuntu questing or what will be 25.10 on release; ie. I have options of LTS or development release), or a secondary box that contains many releases plus a Debian too; that secondary box is much older & thus doesn’t use uEFI.