What's the best option for cutting cement siding?
There is a link to a complete installation guide for this type of fiber cement board at the supplier link in your posting. 

From that guide it says:

Always cut the material outdoors. 


  Never cut the material indoors.
  
  Never dry sweep accumulated dust - Use wet suppression or HEPA Vacuum.
  
  For the outdoor application:
  
  
  Position the cutting station so that the wind will blow dust away from user and others in the working area. 
  Cut with one of the following methods:
  
  a. Best:
  
  
  Score and snap
  Shears (Pneumatic or Handheld) 
  
  
  b. Better: Dust reducing circular saw equipped with a HardiBlade saw
  blade and HEPA vacuum extraction.
  
  c. Good: Dust reducing circular saw with a HardiBlade saw blade (only
  use for low to moderate cutting).
  
  
  NIOSH-approved respirators are recommended for workers when using the
  above recommended cutting practices to further reduce dust exposures.


This is what a cement board cutting shear looks like (click through for amazon link):



This is what a HardiBlade looks like: