He never co-hosted with Imus. They were both on WNBC, and Imus would call into Howard’s show and they did a commercial together. Then when Howard went to KROCK he attacked Imus.
Yup, now I remember. Stern claimed Imus hurled anti Semetic slurs at him and called his sidekick Robin Quivers the n-word, as if Stern never hurled any such insults himself. They both made fun of every conceivable ethnic, racial, religious, sexual, etc group.