Overall, in my experience (as many others will likely agree), providing letter writers as much [concise and straightforward] info as possible will almost always ensure for a better letter. This includes mentioning who else is writing a letter for you (especially if they are from the same institution).
One potential benefit is the letter writer might be able to plan what strengths of yours to focus on or what tone to write the letter based on the knowledge of who else is writing your letters.
For example, if you asked two professors with whom you've previously taken separate courses that you did very well in, they might both be inclined to mention your achievement in their course. However, if you assisted with research with one of the profs and they know your other letter is being written by a professor that will likely discuss your classroom achievements, they will know to focus most of their letter on your research capabilities.
It's also not unheard of for two letter writers that know each other to discuss your achievements/capabilities. This could include, as you mentioned, avoiding unimportant overlap or, more importantly, developing better congruence concerning the "main point" grad programs should be taking away about you.