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...