There’s no real way to have two separate conversations on the same line of code. This has come up a bit recently, actually, where there are two separate bugs on the same line of code and we need to have separate discussions about each of those bugs. Phab definitely has a reply button to comments which is great, but it doesn't have any way of associating those replies with a specific comment (ie, creating comment threads). This would really really really help making comments easier to follow in phab. fwiw, Google’s critique solved this quite nicely with one level of comment threading. I think that part of this feature probably includes adding more context when someone replies to an inline comment:

• Diff emails should include the line of code that was commented on, the previous comment in the thread, and the comment in the thread that was just posted. This way I can read the email and tell exactly what happened. Seeing the author's new comment in the email is definitely helpful, but unfortunately it’s just really hard for me to follow because I don't remember which line of code was on which line in stuff *when just looking at the email*.
• It would be great if the snippets at the top of the “revision” page in phab could include some of these details. I would find this really useful when visiting a diff the *second* time, meaning after I posted some comments and the author of the diff responded. This is a time when I need to get my bearings / get caught up on what’s happening in the diff again, so having some more context for each of the comments would help me out a lot. For context, I am talking about this view: https://www.dropbox.com/s/bs3l965x2kqcdpy/Screenshot%202015-06-17%2000.13.07.png?dl=0 Note how it just shows the comments and the line numbers, it would be great if it could also show the line of code that was commented on and the most recent comment in that *thread*.