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Currently, you cannot combine two user accounts into one. You can however, make one of them your primary. This means that when someone grants you access to a QuickBase (using the email instead of the screen name) or sends an email, it always refers to the primary account.
In order to make an account the primary one, please go through the following steps:
1) Set the email address on both accounts to be the same email address. To learn how please read:
2) Log into the account you want to be the primary account. On the My QuickBase page click the "Signed in as" menu, and choose "Edit User Profile". On the User Profile page, click the "Manage User Accounts" link. Within the Default Account column, click the radio button next to the screen name you want to make the primary account. Click OK to save the changes.