It isn't currently possible to merge separate Todoist accounts. However, if you'd like to transfer your data (active tasks, projects, and labels in use) from one account to another, this may be done using the export/import functionality. Simply back up all data in your account as CVS files and then import them to another account. Please note that your completed tasks, filters, labels not in use, and Karma progress won't be transferred.
Please note that you can also easily change the email address of your account.