Sorry, it is not possible to merge accounts. I'm not sure where you read that. You can manage everything within 1 account if everything originated in that account (same User ID and password), but you cannot combine 2 separate accounts. Here's what TurboTax says about it:
FAQ: Can I merge my Intuit Accounts?
https://ttlc.intuit.com/turbotax-support/en-us/help-article/account-management/merge-intuit-accounts...
