Can you do it? Yes. Would I do it differently? Yes.

I would not bother with keeping 10.10. I would install 11.10 and keep it
simple. If you do not like Unity then you can add just about any DE to the
Oneiric base including Cinnamon from Mint.

But each person is different with varying needs, so what I would do is
unimportant. What is most important in answering this is what is your
purpose in wanting both Maverick and Oneiric?

I think you should be fine. The idea of allowing for growth is good.

The things I'd look out for is boot loader install. In short, I would advise
against installing a second boot loader.

You also should check/ verify that the first OS is detected during the install.

I am not sure of the steps involved but I think a single boot loader ( with
choice of Ubuntus at boot time is possible.

Group? Any thoughts?

Are those primary or extended partitions? You can only have four primary
partitions that can be recognized. That is the only thing that I see
might hold you back.

I only have one primary partition. Actually I did not know there could be
more than one primary.

I'm just experimenting/learning Oneiric/Unity. Somethings I like
somethings, hopefully in the future I will be overcome the "features" I
used, and liked and miss, in Maverick. When the LTS is released I plan on
axing everything but Windows and doing a fresh install.

One of the big things I miss in Unity is having a Top Panel where Icons can
be places that execute a shell script to set up an environment with at
Terminal, Nautilus, GFile and Evince windows open ready to enter code for
programming Atmel AVR micors. Or call up Calc with either my checkbook,
vitals, or portfolio, Python3 programming environment, things like that set
up using one click and the things I use most was ready to use. Now, with
the advancements(?) in Unity I have know idea of how to do that. Any ideas?

See:

www.omgubuntu.co.uk/.../how-to-add-folder-quicklists-to-the-home-laun\
cher-in-ubuntu-unity/

www.upubuntu.com/.../...o-top-panel-of-ubuntu.html

askubuntu.com/questions/38549/places-menu-in-unity

Hope this is what you are looking for.

Here's something you might find amusing. When you have multiple versions of
Linux installed, only one of them will control the Grub. It will probably be the
last one you installed.

If you get a kernel update in a Linux which does not control the Grub, the new
kernel will not be used until you boot into the "master" Linux and run
update-grub.

My laptop is set to triple-boot, so this comes up fairly often.

You can actually work around that grub issue you are describing
if you have grub installed for each os on their respective partitions.
Have all of the secondary grubs set with booting straight into the os, and
chainloaded off of the main grub that you select from. Can be a little
tricky to figure out, but there are probably instructionals out there if
interested.

When replacing Maverick with Oneiric I suppose you delete and re-add and
format the Root partition, leaving the home partition. Is this correct or
more specifically how should it be done?

Or do you just format the Root partition, leaving the swap, and
leaving/setting the old /home partition again as /home but not formating.

Oh, everything has been backed up to a spare second HD.

It depends on your setup.

If you have a separate home partition then you choose a custom
installation. Format root and choose mount point as / and with home you
choose to use that partition and set the mount point as /home, but do NOT
format. Double check before moving on that the only partition set to format
is /. I have done this a hundred times or more. It works.

If you have everything together on one partition then the only option is to
backup home to an external device, including hidden files and folders.
Format everything and copy over home with the backed up data.