I had to log into the site from my phone to see what you are talking about...
The dark chat bubble is for a quoted reply where your post will include the text from the post containing that chat bubble.
The double chat bubble with the plus sign is the icon to add to a multi-quote reply. In a multi-quote reply, you can select multiple different posts by clicking on the double chat bubble icon in each of the posts and the text from those posts will be copied into your new reply with the appropriate 'quote' tags added. This allows you to quote multiple people and reply to each post individually. So basically, you select the posts you want to quote and reply to, then click on one of the quoted reply buttons to get to the post input screen. The icon will change to a double dark chat bubble with a minus sign to show that the post has been selected for multi-quote.