Description by the developers, as quoted by JessicaS,
To multi-quote, first select the posts you would like to quote and then finish it off by clicking the "Quote" button on one of your selections. This will pull those quotes into your reply box. Right now, we don't have the ability to toggle off a single multi-quoted post but if you click the "Clear" button on the reply box, that will clear all of our selections. We're working on the toggle option too!