A further question on my "how to move to an NVME disk" question

[Chris Green]

Thanks for all the replies and help for my original "How to install
grub onto an added drive?" question.

I actually have my nice new NVME SSD and PCIe adapter now and, quite a
pleasant surprise, they "just work". My existing xubuntu 19.04
installation recognises the drive and I've used fdisk to partition it
and mkfs to build a file system.

As an experiment I unplugged all the other drives and tried to install
xubuntu 19.10 on the new drive.  It completed the installation without
any errors but, not surprisingly, the system complains about there
being no boot device when I try and boot it.  So, confirmed, I can't
boot from the new NVME drive.

So, I have thought a bit and I'm wondering if my best strategy might
be to dual boot xubuntu 19.04 and 19.10 for a while, this way I'll get
a 'clean new' 19.10 installation and the old 19.04 installation files
will be available to look at to copy configuration across.

Finally to my question!  How do I simply add xubuntu 19.10 as a boot
option to my existing set up?  As things stand the xubuntu 19.04 OS
and /boot are all on /dev/sdb1 which is a SATA SSD.  So it would make
sense to simply add the ability to boot the xubuntu 19.10 installed on
/dev/nvme0n1p2 to the existing /boot.  How do I do this?

-- 
Chris Green