Step 1. Share the board with your second account
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
Find the account you'd like to transfer content to
Click on the arrow to the right of it
Click "Make you the owner". This will transfer board ownership over to your second account
Now you can log back into your second account and use your boards as normal.