So that I'm clear ... you're trying to quote more than 1 person in a single reply?

If that's the case there's no easy way to do it. The forum was designed to be threaded so that a post is a child of only 1 other post. Add "&mode=threaded" to any thread to see what I'm talking about (use "&mode=linear" to switch back). You can also use the "Display Modes" dropdown to change between them.

The easiest way to do it IMO is if you have a browser that supports tabs just open the "Quote" link in a new tab for each post you want to quote and click "Post Reply" in your current tab. Just copy and paste from the new tabs into your main reply. At least this way it will have the quote tag properly formatted and you don't have to type it yourself.