When these two sort definitions are put together in a single statement a filesort is produced.
Why do we want to avoid filesorts?
Filesorts are bad. 1st they tickle a thread based buffer called sort_buffer_size. Additionally filesorts reads the data twice, unless max_length_for_sort_data limit is reached and as a result the Filesort runs slower to reduce disk I/O. If you want filesorts to run faster at the expense of the disk increase the default max_length_for_sort_data. You can read the filesort algorithm here.
So, here is an example
CREATE TABLE `ABCD` (
`A` int(10) unsigned NOT NULL default '0',
`B` int(10) unsigned NOT NULL