Why would you want to ?

The primary clustering is designed to limit the number of index pages that need to change each time a row updates onto a new data page. In this scenario only the primary clustered index would change.

Adding a second clustering index would force the system to update both of the 'clustering indexes' to point to the correct data page.

If you are not trying to limit the amount of pages touched for Update you could investigate non-clustered indexes on a table without a clustering index (or a heap). As in this case the non-clustered indexes refer to the data pages and not the primary clustering index values. (and in this scenario both would be maintained on each row update onto a new page)