Can the same ArcGIS Online username be used for multiple organizations?

No, the same ArcGIS Online username cannot be used for multiple organizations. A username must be unique across ArcGIS Online. As a workaround, create a new account with a different username to log in to a different organization. Although a username may be associated with only one organization, it is possible to use the same email address to create multiple ArcGIS Online organizational accounts, provided the usernames are unique. These accounts can be linked to different organizations or the same organization simultaneously. This is useful when each account has different roles and privileges within the organization.