How does one add a second Ubuntu installation to an existing one?

[Chris Green]

I want to add a second version of Ubuntu to my existing xubuntu 20.10
desktop machine, I want it to be a dual boot system.

I just want to end up with a boot menu that defaults to the existing
xubuntu 20.10 installation but will allow me to choose Ubuntu (server)
20.04 at boot time if I need to.

To add a slight complication my 20.10 installation has a separate
/boot partition and drive because the main NVME SSD isn't visible to
the BIOS.

So I have the following drives:-

/dev/nvme0n1p2 ext4     48174  15748  29912  35% /
/dev/nvme0n1p3 ext4    896193 295874 554727  35% /home
/dev/sdb1      ext4     10016    183   9306   2% /boot
/dev/sdb2      ext4    109596  33138  70850  32% /scratch
/dev/sda1      ext4    938772 222399 668664  25% /bak
/dev/sdc1      ext4    937872     77 890087   1% /mnt

The new Ubuntu Server installation can go on /dev/sdc1 as that's empty.

What's the easiest (and least error prone) procedure for doing this?

-- 
Chris Green