I have participated in a variety of research projects over the past four years and am having trouble deciding who to ask for recommendations. In particular, I participated in two REU programs, a couple research projects at my university, and have just started working at a national laboratory (after earning my BSc). At each of the REUs, I was assigned two mentors. Combining all mentors (including my new one), I have seven potential recommenders (six of whom I think would write good letters).
For my first REU project, I worked closely with one of my mentors and had little contact with the other; however, the other mentor is more well-known. For the second REU, I worked just as closely with both mentors, but one is again more well-known.
Would it be possible and/or advantageous to have each pair of REU mentors write "joint" letters on my behalf? Would it be better to choose the mentors I worked more closely with or the mentors who are more well known? If it were possible to have them write joint letters, this would bring me to four strong recommendation letters total.
Any advice would be appreciated!