Or:
1) Open one of both files.
2) Export all transactions to csv file (this is existing functionality)
3) Close the file and Open the other file
4) Import the exported csv file

Job done.

For starting with something simple to avoid that would work:
1) Open one of both files
2) Import the other file
The accounts from the imported file could all be in one (or one per account type if need) subfolder
3) I can reorganize the new account hierarchy now as I need it

This simple solution can be implemented without a GUI beyond file selection for the file to be imported (?)

But what should happen if those two files contain account names that would clash? There sure needs to be a giantic new user interface to handle all potential clash situations, doesn't it?