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