Can both a grad. student & faculty member cosign a recommendation letter?
Like Noah, I had a situation where two advisors co-signed a letter of recommendation. I should mention that the people reviewing the letter found this an unusual situation—and had claimed that they had not seen that in twenty years of reading recommendation letters. So this is definitely not standard practice. I suspect it would be memorable, but I am not sure it would be actually useful.

However, the difference was that my two co-signers were equal in rank. Your situation has a professor with a graduate student providing most of the insights. I suspect you will need to have the professor adapt the graduate student's comments, and then sign the letter. In the case where feedback is needed, the professor would then need to get the relevant details from the graduate student.