You can consolidate multiple companies under one user ID. This must be accomplished through the user list with invitations. It is a multi-step process to complete the consolidation.
If you try to simply change the user ID within the  Your Account > Personal Info section, you will see a message that the user ID is already in use.

To accomplish the consolidation, follow these steps:

First, send and accept the invitation:

  1. Sign in to the company that you want to consolidate to your current user ID.
  2. Click the Gear Icon and choose Manage Users.
  3. Click New.
  4. Enter the email address associated with the user ID and click Next.
  5. Select Company Administrator and click Next.
  6. Click Finish.
  7. Sign out of the company.
  8. Check your email for the invitation.
  9. Click "To accept this invitation, click here".
  10. Choose the option "Already have an Intuit Account" and sign in with the same credentials of your other company.
  11. Once successfully signed in, click sign out.

If you are the Master Administrator of the company for which you are changing the user ID, you will need to complete the following additional steps:

  1. Sign in to the company using your current user ID.
  2. Click the Gear Icon and choose Manage Users
  3. On the bottom left, click Transfer Master Administrator.
  4. Select the new user ID and click OK.
  5. Sign out.
  6. Check your email for the Master Administrator transfer notification.
  7. Click Accept and sign in to the company.

Note: The Master Administrator role will not be transferred until the new user successfully signs in to the company.

Once you have completed the setup of the new user and have the access switched, delete the old user:

  1. Sign in to the company using the new User ID.
  2. Click the Gear Icon and choose Manage Users.
  3. Select the old user to remove from this company and click Delete.

Note: Now when you sign in to QBO, all companies which are associated with your user ID will appear so that you can easily choose from among them. To switch between companies once they've been consolidated, go to the Gear icon and select Switch Company.