Unfortunately, it’s not currently possible to merge separate Todoist accounts.
If you'd like to transfer your data (active tasks, projects, and labels in use) from one account to another, you can do so by using the export/import functionality. Simply backup all the data in your account as CSV files and then import them into to your other account.
Please note that your completed tasks, filters, labels not in use, and Karma progress will not be transferred.
Alternatively, you can change the email address of your account.