Did you want music for a flute and clarinet duet? Or solo music for each instrument?
When you say preform do you mean to rehearse some numbers before the actual performance? When one preforms something is prepared and finished in time for its performance. Other than that, my answer is a waste as I know next to nothing about Catholic music. I'm a Latter-day Saint and I recommend Come, Come, Ye Saints on the clarinet. But you may try Amazing Grace on either of the instruments and it will be beautiful.
