Pair programming is an agile software development technique in which two programmers work together at one workstation.

Pair programming is an agile software development technique in which two programmers work together at one workstation. One, the driver, writes code while the other, the observer or navigator,^[1] reviews each line of code as it is typed in. The two programmers switch roles frequently.

https://en.wikipedia.org/wiki/Pair_programming 

Pair programming is quite famous now-a-days.

It has several advantages like:

1.      Programs with fewer bugs.

2.      Post production maintenance cost is much less.

3.      Established practices are challenged resulting in emergence of new ideas.

4.      Programmers learn from each other.

5.      Programmers develop soft skills.

Although pair programming has gained considerable reputation, it has several pitfalls too.

Some of them are as follows:

1.      In pair programming you cannot sit back and self-evaluate your own code.

2.      One of the pair may stop being actively engaged.

3.      The driver needs to "program aloud". Silently programming reduces the benefit.

4.      It costs more man-hours to produce the same features. Balance must be maintained between quality of code and increased coding cost.

5.      A "watch the master" phenomenon may arise when an experienced and a novice programmer pair up. The novice member may become the observer with the experienced member completing most coding.

6.      When two experienced users pair up, a "developer's ego" phenomenon may arise, with each member trying to push his/her own ideas.