How Do Alts Work?

Basically on the launcher bellow your mcname there is the choice to switch users, just insert your email and password for the new user and afterwards that new name will always appear on the list of "existing users" you can choose to play with, no need to insert your login info again, unless you log out of that account.

You would need two launchers if you happen to have your launcher settings to close once the game starts, but there is also the option to keep the launcher always open on "edit profile" on the right side of the launcher > "launcher visibility" options. If it's always visible you can launch one game, then switch users with the same launcher (since the launcher didn't close upon game start) and click play to start another game with the new alt, and that way have two mc with different accounts open at the same time.

It's not possible to set up a launcher with always one username and another launcher with the other name, since upon starting a launcher it always displays the latest account you played with, you have to switch users in order to change accounts - but hey, at least you don't have to insert your login info each time ᕕ( ᐛ )ᕗ

Then in game you can use Wynncraft's trading system to store items on the alt :)