What is the best way to export/import transaction from a Gnucash file to another Gnucash file?
The most obvious (and actually only way I can find) is to Export Transactions to CSV... and then to import from your other account. But CSV import doesn't allow to import all of Gnucash information categories, and at best can only recreate basic transaction.
For instance in the exported CSV, all the splits are included, and actually all the details of each every transactions. But when you try to import such a file, you get errors for all the lines that don't include Dates (all the splits) to start with, and so on.