To merge decks from one account into another, please follow these steps.
1) First, sign into the source account and make your decks public and sharable. Look for the lock icon in the top of the editor. There's a little checkbox under privacy that you can select to make all of your decks sharable. (more on privacy settings here)
2) Next, email yourself a link to each of the decks from the old account or copy/paste a link to the decks using the copy link feature in the share dialog box (more here)
3) Next, sign out of Haiku Deck and sign back into the destination account. Open the decks from the old account (remember those links from emails from step 2?) and click the COPY DECK button below the playback viewer. This will make a copy of your deck in the new account. Repeat for all of your decks.
4) Lastly, if you wish to close your original account (the source account), click this link to learn how.