Such a comparison is useful for revelation of distinction in keys similar by structure but different in names - paths, for example:
HKEY_USERS\.DEFAULT and HKEY_USERS\S-1-5-18.
Such a comparison is possible to do in one copy or in different copies.
To compare the selected key with the other one press Compare with other key button. After you see the message
"choose the second key for comparison", it has to be already opened, because ARM will choose the next key selected by you for comparison.
Now you have to defined the main ("root") key from these two keys, i.e. the key about which the results of comparing will be distributed in the "Compare window " - the keys, which will appear in the "Added" and in the "Deleted" tabs.
After that you will see the window with the results of comparing.