How can I flag a person who is trying to earn dummy reputation?
Today I have seen 2 people who are doing something along the lines of this: One is asking and the other is answering and vice versa.
How can flag this kind of activity?
Flag one of the posts in question and use the 'Moderator intervention Required' reason, and add some reasoning in there. Mods have tools at their disposal to investigate things like this.
If you are really sure, flag one of the posts of this users with the "in need of moderator intervention" flag. Be sure to write as much as you know as possible. For example:
I think these users are just asking and answering to farm reputation. They have been doing this at <this>, <this>, and <this> post.
Where every 'this' is a link.
Also, a good thing to check whether you are right: Is the answer of the other user of the pair usually accepted?