Move Viber.exe - Shortcut to C:\Users\\AppData\Roaming\Microsoft\Windows\Start Menu.
Right click on C:\Users\test\AppData\Local\Viber\Viber.exe -> Create shortcut.
How to add it to start menu and run it w/o entering passowrd (optional) If you want to avoid that, add user without passowrd, than change passowd via control panel. Note that, creating user with password, requires adding answers on questions.
Next time you want to start already installed viber: Shift + right click on C:\Users\test\AppData\Local\Viber\Viber.exe -> run as different user.
Select test user, enter password, install viber
Shift + right click on C:\Users\test\Downloads\vibersetup.exe, and select Run as different user.
Download vibersetup.exe and move it to C:\Users\test\Downloads.
After Windows finished preparations, switch back to your primary user.
Log in to test, so that Windows can create files for that user.
Create new local user, say test ( with password).
I managed to run several Viber instances (and actually, any app, that is supposed to be run in single instance) in one user session.