def join_rows(row_list_1, row_list_2):
    """Returns a list of list, in which element is the concatenation of the 
    elements in the same position of row_list1 and row_list."""
    
    if not row_list_1:
        return row_list_2
    if not row_list_2:
        return row_list_1
    row_list = []
    for (row1, row2) in zip(row_list_1, row_list_2):
        row_list.append(row1+row2)
    return row_list