You open one thread in two browser windows/tabs. In window #1, click on the Post Reply button, in #2 click on the quote button of the first reply you want to quote, copy the entire editor contents into #1, then click back and click on "Quote" for the next comment you want to respond to, copy and paste it into #1 (after the first pasted text), and so and so forth - until you have all the replies quoted that you wanted to respond to.
In the end, write your response(s) at the beginning/end/between the quoted posts in the #1 window/tab and hit Submit. Voila!