It's nice to see you again in the QuickBooks Community forum. I'll provide steps to make sure you're able to link your two Quickbooks account with the same login credentials.
Once both accounts have a different email address or user ID, you can invite the other company file to use the same email. You'll have to open your one QuickBooks account and choose the email address you want to use for all your companies. For example, we call this email A. Then, the email you no longer wish to use is email B.
Here's how:
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 choose a user type. Then, select Company Admin if you want email A to be able to manage other users and company info.
Click Next and tick All. Then, answer the following prompt.
Enter your name and the email that you want to login moving forward.
Hit Save.
Once done, you'll receive an email notification. Then, make sure to enter the same password or credentials. Once configured, you can now see a switch function (Switch company) under Profile in the Gear icon.
You can always get back if you have other concerns. Please let me know by adding a comment below. I'm here to help you achieve your tasks in QuickBooks.
Thanks for following the thread. I'm happy to provide an additional way to link your accounts in QuickBooks Online.
Aside from the steps provided by my colleague above, you can change your email and user ID in your second company, and the accounts will automatically link. I've included some steps to change your email below.