CellRange("A1:A5").value + CellRange("B1:B5").value
Will add together two lists of length 5 to give you a list of length 10. To concatenate the elements in the lists, you should do:
CellRange("C1:C5").value = [x + y for x, y in zip(CellRange("A1:A5").value, CellRange("B1:B5").value)]