Multi farm block: Allows you to farm on a maximum of 2 characters only. (This restriction applies only to farming: you can be logged in with as many characters as you wish.)
Multi-dungeon block: Allows you to receive rewards in dungeons on a maximum of 2 characters only.
If you logged into the game with multiple characters, you can choose which one you want to use for looting and entering dungeons. You can easily do this by clicking on the icon in the top right corner. You will see this icon when your character can enter dungeons without restrictions. At this point, you have nothing to do.
If you log in with multiple characters, you will see this icon for the 3rd character and onwards. You can choose to deactivate it with previously logged-in characters and activate it with new ones as you wish.