Can't log into two or more premium accounts at once. Once you log into one, and open a second launcher it will result in a bad login for the one that was opened before, even if they are both premium.

Replicate:
1- Open launcher and log into a premium account.
2- Open a second launcher and log into a different premium account.
3- Try to log into a server with the first opened account.
4- Results in a bad login.

This is very annoying when needing to use two accounts for testing or administration on servers. The only way to get past this is to log in with the first account, then open the second launcher. But when a server restarts, it makes users have to close out of minecraft and open again just to log into a premium account over and over. The client should support multiple premium accounts at once.