'costruisco la relazione
Dim relAB As New System.Data.DataRelation("relAB", ds.Tables("A").Columns("AID"), ds.Tables("B").Columns("AID"))
ds.Relations.Add(relAB)

DataGrid1.DataSource = ds
DataGrid1.DataMember = "A"

(ds=il mio dataset)

...forse nn faccio abbastanza?