While contacting professors should we mention if we have contacted some other faculty in same department? If a student is contacting faculty member and finds that there are two professors that are of his interest. So, he decides to contact both of them.

While contacting should the student mention that he has also contacted another professor in the same department?
I'm assuming this is regarding admission to graduate school. I don't see any harm in mentioning this as a courtesy. The odds are high that if the two professors work in related areas they will discover this themselves. If you haven't mentioned this to them, it will seem as if you're flooding the department with applications. 

It also helps personalize the letter, which is the number one thing you have to do if you expect to get any kind of response.