try

select /*+ ORDERED */ a.file_name, b.phyrds + b.phywrts "Total I/O"
from dba_data_files a, v$filestat b
where a.file_id = b.file#
union
select /*+ ORDERED */ a.file_name, b.phyrds "Total I/O"
from dba_temp_files a, v$filestat b
where a.file_id = b.file#
order by 2 desc