First and foremost, before doing anything on the database level make sure you have a recent backup (and have verified that you can actually restore it). If possible, create a new backup before you start changing things in the database.
For merging 2 users in the database you'd identify all tables that have user ID information in them (most of them do):
SELECT DISTINCT table_name
,column_name
FROM information_schema.columns
WHERE (column_name LIKE '%userid' OR column_name LIKE '%user_id')
AND table_schema = 'FOOBAR'
AND table_name != 'qa_users';
Replace "FOOBAR" in that query with your actual database name.
Then update the fields containing the ID of the second account with the ID of the first account.
UPDATE qa_blobs SET userid=23 WHERE userid=42;
Replace 23 and 42 with the correct user IDs (and of course adjust table and column names as needed).
You may want to wrap all updates in a transaction, so that a) you can roll back if something goes wrong in the process, and b) users and applications won't see modified data before the transaction is completed.
BEGIN TRANSACTION;
UPDATE qa_blobs SET userid=23 WHERE userid=42;
UPDATE qa_eventlog SET userid=23 WHERE userid=42;
...
COMMIT;
In the web interface verify that the merged user (ID 42 in the above examples) doesn't have any posts associated with them anymore, then delete that user.
I'm not aware of a plugin that would simplify these steps.
