Is it possible to belong to more than one ArcGIS Online organization while using the same user name?
It is your username that has to be different. In my case, I have two user names and belong to the same organization because I have different roles within there and different priviledges. More importantly, one of the names comes with me when I retire the other doesn't
well it is a good idea too... but you can have separate accounts and I would separate your esri account from a work account for sure check the docs in the workflow and design section What's new—ArcGIS Online Help | ArcGIS
Are you using a built-in username?
If so, i would think ArcGIS Online usually matches usernames to organizations based on its unique value when logging in. This is why: you can still login to www.arcgis.com using your built-in username and password and still login to the right organization. I am not sure if you would be able to use that same name in a different organization.
Currently, one ArcGIS Online username can belong to one ArcGIS Online organization. One username cannot belong to two organizations. You can reuse your email address for many usernames.
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