In the Kobold file, I found a file called Kobaxe3.ATT. In it, there is already an entry called RIGHT_HANDED. It's value was 1.00000, so I changed it to 0.00000.
However, there was a variable missing that I found in an existing two-handed creature's ATT file, called TWO_FISTED. Once I put that in and set it to 1, I repacked the ATD file and played an arena match. I found that it actually works.
I can control both arms, but the only drawback is that the Mouse VSIM doesn't work with two-handed mode yet.