How to make a multiple select drop down list

How to make a multiple select drop down list

Hello, I would like to know how to make this ( http://www.atlassian.com/resources/better-together ) 2 drop down lists which makes content appear depending on both 2 selected items in the drop down menus, is there any documentation or already made plugin so I can use to achieve this?
Please I need help!!!!
THANKS!