In songwriting it’s not unusual for lyrics to be coauthored, and it’s easy to come up with many famous examples. (Lennon and McCartney, for example: though some material credited to both was actually written only by one or the other, many other lyrics were written jointly, or with significant input or revisions from the other.)
However, I can’t recall any famous poems which are credited to multiple authors. Are there any? To be clear, I’m talking about actual collaboration at the writing stage, not one poet’s independent translation or adaptation of another’s work, nor one poet’s completion of a deceased poet’s incomplete poem.