SUMMARY
Add the option to compare two separate album selections and add the option to group album selections by root album when searching for duplicates.

*Compare two separate album selections: Instead of one selection of many albums that are compared against each other, you have two selections of albums (two dropdown selectors), and each selection is treated like a single album when setting search restrictions.
*Group album selections by root album: After you select albums to compare, it finds the "root" album selections. Root albums include any selected album that doesn't have a parent/grand-parent/etc that is also selected. It then treats any selected children of a root album as part of that root album when setting search restrictions (instead of treating them as completely separate albums).

The two selections or the two (or more) grouped root albums can be compared against each other without comparing photos within themselves ("Exclude reference album/group/selection"). They could alternatively be compared to themselves without being compared to each other ("Restrict to reference album/group/selection").

STEPS TO REPRODUCE
1. Under "Similarity->Duplicates", select two albums and their child albums
2. Enable either "Restrict to reference album" or "Exclude reference album"
3. Search them for duplicates

OBSERVED RESULT
Every album (including child albums) is treated as a completely separate album when applying search restrictions ("Restrict to reference album" or "Exclude reference album"). There is no way to treat multiple albums as a single album for finer control of the search restrictions.

EXPECTED RESULT
When using search restrictions ("Restrict to reference album" or "Exclude reference album") on a duplicates search, you should be able to either make two separate selections (using two separate dropdowns) that are both treated as one single album when applying restrictions or be able to have the children of an album be treated as part of that album such that each top-most-level album and all of its selected children are treated as a single album when applying restrictions.

ADDITIONAL INFORMATION
The standard search where every album is independent, the search using multiple selections, and the search that groups by "root" / "top-most-level" album would all be mutually exclusive and you would select your desired "mode" using a dropdown.

If either "use multiple selections" or "group by root/top-most-level album" were the selected mode, it would require you to select one of the restriction options; either "Exclude reference album" or "Exclude reference album" because otherwise, it would produce the same result as if you didn't turn either of them on.

Also, the "Exclude reference album" and "Exclude reference album" options would change their name depending on the selected "mode", or they could just be statically named "Exclude reference album/group/selection" and "Exclude reference album/group/selection" respectively to account for all modes.