The reason I have seen stated is that the load from a double rope will be partially shared by the second rope. This reduces the force on the first strand. The physics is good but idealized. I have always been suspicious that this may not be the case in the real world, but I have no proof.

Respectfully,

Jerry

If the load is shared, you either have a protection/bolt very near to each other or you need to have find a new belayer. Thats my experience anyways.

Never mix twin and double technique, which will share the load, but one might damage the other.

Friends in Britain in the 70's said people climbed double rope on the sharp rocks using a 10mm and a 9mm. At the time the US standard was single rope 11 mm with a tail rope of 9mm for rappels.

Who uses double rope or twin rope technique? Most of my climbing has been in North America and I have never encountered either technique...

Alpinists mostly.

Double rope for
Winding routes to reduce rope drag,
Belaying two followers at the same time on easy routes like Mt Whitney east
face, for speed and reduction of rock fall hazards or for training a new climber on easy terrain when paired with a experienced second.

Twin ropes for
ice climbers-lighter weight when you need full length rappels, less chance of cutting both ropes with axe or crampons.
Sport climbers working a route as twin ropes hold more falls before failure.

In the old james bond movie with the full length climbing fall scenes, I was told stunt man Rick Silvester used a pair of old ropes and tied them together with rubber bands to make it look like one rope so he wouldn't ruin a new rope. Effectively using a twin rope technique with old single ropes.

http://blog.owareusa.com/2014/01/24/castle-peak-on-donner-pass-california/

I know that's what the manuals might say,but I have never seen it in use, particularly in placing leads, including the east face of Whitney, which I have done. Belaying seconds is fairly trivial; I should have been more clear that I was particularly concerned with leading situations.

I never really gave twins and doubles mucht tought, untill I climbed a pretty zig zag route on trad gear. I ended up pulling over half of my protection of the wall during the climb...

And doubles/twins are pretty much mandatory in some longer multipitches with seperate 'abseil pistes'. But i have only climbed in europe, so bolting pratices is dependend on location.

As you would know, on long traversing pitches, having gear placed to protect the third as well as the second can take some thought, either by the second clipping the rope for the third in as he passes or by the leader using some twin or half rope methods and clipping the pieces for both climbers as he leads.

Twin ropes have some limitations on belaying a follower on just one rope (generally only used for skiing, glacier travel, low angle terrain etc.) where the impact force of a fall would be minimal. And as mentioned, don't mix twin and double rope technique in the same pitch. So belaying second and third climbers can take some thought.

Here is a shorter single rope at REI that is a great value for someone who need a tough high strength climbing rope of a shorter length and less weight.

http://www.rei.com/product/884526/maxim-equinox-99mm-x-35m-gym-rope

Just found some more short ropes.

http://www.gearexpress.com/climbing/rope/short-rope.html

If you want short ropes, you can also just go to a climbing store and buy per feet of the rolls they have.

9.9mm is not light nor thin for a single rope. The thinnest single rope rates ropes are 8.7mm. (ofcorse with reduced durability and lower friction in belay devices).

If you use rope with belay devices and carabiners, try to keep the rope clean. Dirty rope wear down equipment fast. Impregnated rope help with keeping rope clean (and drier = lighter).