About SQL Server - Server + CAL license _ Part 2

Hi, in below link Olaf has confirmed that if I have server license then i can have multiple sql server installations on the same server (means multiple sql engines). Suppose with 1 server license, on a vm/server, I install 2 sql servers (instance A & B) and users are user1, user2 and user3, common for both instances A & B. So do i need 6 CAL licenses or just 3 CAL licenses?

https://social.technet.microsoft.com/Forums/en-US/a579b3ba-8293-48a3-b0c4-4dec13a65c28/about-sql-server-server-cal-license?forum=sqlsetupandupgrade

thanks in Advance.

Chirag