Which group of chordates has a notochord and includes separate males and females?

Lancelets is the group of chordates that has a notochord and includes separate males and females.

Lancelet exhibits the four basic characteristics of phylum Chordata. It has a dorsal nerve, a notochord, a post annul tail, and pharyngiall gill slit.

Jawless fish, cartilaginous fish, and bony fish have notochord when they were still young but it was replaced by a vertebral column as they mature. 

Lancelets are included in the subphylum cephalochordata. Cephalochordates are gonochoristics. It means that the sexes are separate. Both sperm and eggs cells are released in the water where fertilization occurs.