I have a question regarding recommendation letters for PhD applications that I would appreciate if anyone could help me with. I am a research assistant and have been working with a young economist who has made it clear that he doesn't think his recommendation would carry much weight. Our boss suggested the idea of having the two of them co-write a letter of recommendation. I've worked on policy projects for both of them and on research with the younger economist, so I believe they will have non-overlapping information on my skillset.
Our boss got his PhD ~15 years ago and the younger economist about 5 years ago so they are both not sure if this is generally accepted. Is there a rule of thumb for whether or not this is a good idea? Thanks in advance!