• One Solution Can be use of Lookupset function:

    use this expression in one cell

    =String.Join(VbCrLF,

    Lookupset(Fields!joinFieldName.Value,

    Fields!joinFieldName.Value,

    Fields!FieldYouWant.Value ,"secondDataset")

    )

    It joins firstDataSet (current) to second dataset and feed result in every new line.

    Excel output is also well!