I love double weapons and have been thinking about these questions for a while. Full size shuang jian are extremely rare, while full size shuang dao are only slightly more common. The examples of these that I have seen are all late 19th c. or later. Earlier examples of both jian and dao tend to have had blades around 20 inches or shorter and frequently show signs of substantial use. These older examples tend not to be in as good condition. I have a book of woodblock prints showing Ming period soldiers using various kinds of siege towers that shows a substantial proportion using two willow-leaf shaped swords. There are several woodblock prints showing these types of soldiers, but they are the only examples I have seen. Usually double weapons are a sign that the user did not belong to the regular army. The woodblocks are interesting to note for their very specific depictions. The soldiers with two swords were part of groups, preparing to clear defenders from walls. I suspect that in regular massed combat, double swords could have been too dangerous for ones own side. They seem particularly good for wading into masses and having at it, rather than being part of a mass fighting another mass.
Also it has been noted that there would have been a considerable surprise element for a double weapon wielder. This would make sense for a caravan guard or someone working for a security firm, but no one would run screaming if they were calmly looking out over an apposing army and one of them suddenly produced two swords. As further evidence of the lack of the need for surprises of this sort when part of a large army, it can be seen in the wood block examples that the swords have full round guards. Thus they were much more likely to be simply a matched set rather than true shuang dao. Surprise was not the issue, clearing the wall was the issue.
Unfortunately, matched sets of willow leaf or other dao have probably all been split up and might not be recognized even if they did exist. I saw a matched set of training dao once that was split up because it was not understood that the swords might belong together. I do have two sets of sword breakers that are sets, but not designed to fit in a single sheath. I bet there used to be more sets of different kinds of weapons that are now split.
Josh
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