I think an important concept to bring up also is the importance of armor in the war doctrine or psyche of a group of people. Europe has always been relatively big on armor for whatever reason. You can go back to the days of Hellenic hoplites, Iberian caetrati with bronze plates, Celtic warriors (some naked, some bare-chested, some with mail), Roman maniples, etc. While the majority of the people did not have armor, those who could get it often did. While cavalry was key, while the Carthaginians and Successor Greeks loved to use elephants, and while skirmishing infantry could seriously damage morale, the European fighting mindset seemed to be very focused on the core being well-armed infantry. Two armies would line up and wear each other out through better armor, tactics, use of formations... and attrition. This is evident even when gunpowder was used, with long lines of musketeers blasting away at each other until one line was obliterated or lost resolve. Most european tactics focused on this mindset. There have always been exceptions, such as Hannibal and Napoleon who practiced strategies involving much more mobility and flexibility.
If you read up on Eastern strategies and war philosophies, maneuverability and swiftly striking vulnerable targets, and positioning seem to be emphasized much more. While heavier troops always existed... armor rarely reached the same level of popularity, and mobility was always important - from the steppe tribes, to the rattan armored southern Chinese, to the Burmese, Thai, Melayu, Dayaks, etc. Of course environment also affects this as mentioned before.
I feel, though I may be wrong, that there's definitely a negative correlation between armor and two-weapon use. However, I also think there is a greater amount of skill required in the handling of two weapons and especially when defending - where-as with a shield it is somewhat simpler. Also, when fighting in masses in formation (which not all SE Asian peoples did), shields can be interlocked for shield walls and useful for all sorts of formations... dual weapon troops may hinder tight formations because they'll be swinging swords from both sides.... ?
I don't think it's been mentioned, but rattan armor was somewhat common in southern China and Taiwan... rattan was decent protection while allowing for flexibility, mobility, and breathability. And I also dimly recall Khmer troops wearing two chains across their chests... ?
Just some thoughts, hopefully it adds a new perspective to this excellent discussion so far.
