Might you explain what you mean by a subset?

I have a list of over 120 items in alpha-order. One of those items points to
another list that has more than 20 items. Currently I just link to the 2nd list
from the main list. I want to add the 2nd list to the main list and have it
collapsed, but expandable with the + sign.

Open the lists in Datasheet view and copy-paste?

Copy List 2 into List 1. Create a column to identify content in the orignal
list 1 as opposed to the content from list 2. Group based on this identifier

Is this what you mean?

www.zimmergren.net/.../sp-2010-how-to-relational-lists-in-\
sharepoint-2010.aspx

I assume there an exact common data item (column) between List A and List B.
Put a ListView Web Part for each list on a new aspx page saved in some DocLib.
Then configure "connections" to send from A to B. All of this is done through
the UI.

When you view the page, all items from A and B will show up, however, there
will be radio buttons on items in List A. When you click one of these, ONLY
the corresponding (linked) items from List B will appear.

Is this what you want?