Thank you for posting here in the Community. I can help you set up the QuickBooks accounts using the same login credentials.
You can use the same login and email address to sign in to all of your QuickBooks Online companies. For the steps to be more understandable, let's call this email A (the email you want to use for the same login). The email you no longer want to use is email B.
To start, please follow these steps:
Sign in to one of your QuickBooks Online companies that you access with email B.
Invite email A to the company. When you invite email A, you select a user type. Choose Company Admin if you want email A to be able to manage other users and company info. Note: If you can’t invite other users, you need an admin user to send the invite.
Go to your inbox for email A and click the invite link you received there. Email A can now access the company.