Obviously I didn't say you should reply in a row (post in a row), I said you can post provided you didn't break the rules of the forum which means after a forum user have commented you can reply.. E.g Mr A commented and you have something to reply him, you can reply him. Mr B commented and you have something to reply, you can also reply him or you wait and reply them both in one reply. Both posting pattern is accepted and you didn't break any rules when you reply in both ways but if you reply in a row, both replies will be merge by a moderator.
There is no agreement on how to post for replies.
If I am a user makes a reply, above my posts, there are some posts that I want to discuss, I can quote all of them and reply them in my single post.
If I make a reply, after that reply, another user comes with a post, I can quote his post and make my new reply. Here, I have another choice to reply, quote it and edit my previous reply.
Exactly what CryptopreneurBrainboss has said above. All are saying same thing from different directions. The rule is that you don't post consecutively. But if someone replies after you, you are free to drop another post. You can also post consecutively if you quoted someone in the first reply and then reply something independent in the second reply. If I'm wrong I should be corrected.
Both styles don't break forum rules but posters often choose a first style than a second style.
It depends on the board. In meta for instance, the second style is mostly used. But in gambling discussion mega threads, people tend to use the first, especially people who has high posting requirements.
