Blutarsky on "Insert post between posts"

I am attempting to use WP to manage a sort of conversation between two people. Each member in the conversation may post a "reply", hence the WP site on the index page will load all posts toe resemble a conversation.
The problem will occur once I'd like to insert a post (comment) between two existing posts..... is there a way to achieve this?
For example:
post 1
post 2
post 3
.....

How to insert a post between 2 & 3???