I believe it is technically possible to have the same user logged in on multiple machines concurrently (there may be Configuration settings that impact this, but I really do not know off the top of my head). You could always test this to be sure. I just know I've seen the same user logged in multiple times, with different IP addresses, when reviewing some of the data in my Admin panel.
However, if you're planning to Share login accounts across multiple users, it is most likely going to break the rules laid out in your End User License Agreement: End User License Agreement | Tableau Software
Tableaus server has a Guest mode if you have a core license. In this licensing mode, you could have multiple guest users have access to visualizations In a secure way - if you configure it that way.
Generally speaking, Tableau delivers enterprise level security - so depending on your use case, you can configure Tableau server to deliver secure access and authorization to users and data.
Unless you have guest access enabled, you must have your own named user to access the server and data.