In reply to Happeuss:
One handed climbing drills are brilliant for coaching, I regularly get my students to traverse and top rope one handed; It develops timing, accuracy and focuses their minds on the importance of balance and their awareness of centre of gravity.
For slab climbers it's a great way to improve technique, some slab climbs are almost as easy to climb one handed than with both!
My hardest one handed lead was a 7a+ short and technical slab in Squamish with all the clips made with the same hand.
Seamstress in the slate quarries was a fun one handed solo many years ago when I was stupid enough to solo on the slate.
It's worth mentioning that there are climbers out there who have no choice but to climb one handed due to disability, and their achievements are infinitely better than ours....