Step 1. Share the board with your second account
Log in to the account which contains the board you want to move (e.g. firstname.lastname@example.org)
Navigate to the board board you want to transfer.
Open the Share popup and invite your second account (e.g. email@example.com) to edit the board.
Step 2. Transfer board ownership
Once your second account (e.g. firstname.lastname@example.org) has been invited to edit the board, click on the avatars that appear to the left of the "Share" button:
This will reveal a list of all editors on the board; look for the account you'd like to transfer content to, and click on the arrow to the right of it:
Click on the option to remove them from the board:
This will transfer board ownership over to your second account.
Now you can log back in to your second account and use your boards as normal.