I haven't tried it myself yet, but I believe you click on the multiquote button (furthest right, bottom right) and go through the thread clicking on as many quotes as you wish. I'm sure there's a limit. Edit: just tried it out on this thread. After choosing which posts you want to quote, press "Reply with Quote".
Here is what I did to make it work - on first post click multiquote on second post click reply with quote. Then you have it and can type between them, edit them(get just part interested in replying to), etc.
One word of caution when using it now. Be sure to uncheck them when done. Elsewise, if you post with a quote again in the same thread, you will requote all that you still have checked.