As in the subject: I have two computers that I would like to have identical
local mail folders (for obvious reasons). Because of the mess between
akonadi.db, file_db_data, .local-mail.directory and local-mail this is very
difficult.

What is the correct way to synchronize local mail folders between
two computers?

Or is this impossible with kde + kmail?

The Akonadi database / file_db_data files should not be synced.
The ~/.local/local-mail and ~/.local/.local-mail.directory are the directories you want to sync for your local email.