One of the combatants in my game is a dual-weapon wielder. What's the procedure for that? Should I use GM attack for attacks after the first?

First of all thank you for purchasing the Hero Combat Manager.

 

I apologize that the documentation isn't clear on how to accomplish.

 

You should use Autofire for characters that are intending to use multiple attacks against a single target.  I will work on something that makes this more clear on the initial attack tab.