I would allow the new OS to install bootloader to sda, and after installation is complete, restart and edit:
/boot/efi/EFI/ubuntu/grub.cfg
it looks like this:
Code:
search.fs_uuid 77e5c27d-4818-4b8f-b38a-597dcd57bb7e root
set prefix=($root)'/grub'
configfile $prefix/grub.cfg
Change the uuid in the search.fs_uuid line to the uuid of the root partition of the older install. Save the file and restart. System will boot to the older sda5 install. Run sudo update-grub to add the new OS to the grub menu of the older OS.
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