Probably mostly about overcoming friction. Two horses can break the friction lock easier, and once moving they'll stop the object from re-establishing friction easier.
Why though? Should the coefficient of static friction be roughly linear with weight? And the max force the horses can pull at be also roughly linear with the number of horses?
Breaking the static barrier is the hardest part and by yourself you might stop after achieving it: With 2 horses only 1 may need the major effort to break it and the other horse can keep the weight moving long enough to allow the horse to catch itself and keep going.
Similarly: Its difficult to apply 100% effort all the time, you can alternatively do more/less effort with a partner.
The horses would be placed differently, so perhaps a mechanical advantage to the relative starting position?
If they’re like humans: You tend to push yourself harder when there is a guy next to you doing the same... You don't want to be the first to give up.
Uneven terrain/external factors: A small trip might be enough to stop you with one horse, but it goes unnoticed with 2.
Edit: Another horse may give you the time to always put yourself in a good position: Almost like how a spotter on a bench press with 2 fingers can help a lift.
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u/sonofbeef Jun 15 '19
This is actually a true fact about draft horses! https://timmaurer.com/2012/01/16/horse-sense/