In odoo we can manage multiple POS users different ways,

1. Switch Cashier without any security

As prerequisite, you just need to have a 2nd user with the Point of Sale User rights (Under the General Settings ‣ Users menu). On the Dashboard click on New Session as the main user.

On the top of the screen click on the "Lock" . button

And switch to another cashier.

The name on the top has changed which means you have changed the cashier.

2.  Switch cashier with pin code

Configuration
If you want your cashiers to need a pin code to be able to use it, you can set it up in by clicking on Settings.

go to employee profile update -> HR Settings -> Update PIN

On the Dashboard click on New Session.

On the top right of the POS screen click on the "Lock" . button

Choose your cashier,  You will have to insert the user's pin code to be able to continue.

Now you can see that the cashier has changed.

3.  Switch cashier with cashier barcode badge

If you want your cashiers to scan its badge, you can set it up in by clicking on Settings.

go to employee profile update -> HR Settings ->Generate Badge

On the Dashboard click on New Session.

When the cashier scans his own badge, you can see on the top that the user has changed.

4. If you want to access same session  for  multiple users at  same  time ,

it's possible 

First You need to open a session as POS User 1,

Then if the second user would like to access the same session ,

In a separate system  login as  second  user -> Open POS-> go to ->Orders -> Sessions -> Then click  " continue selling".

https://www.youtube.com/watch?v=d8qO9PVglx0&t=12s