My brother and I just finished this past summer two sides of my father's 36' x 96' barn in cement fiberboard (hardie). What we did was set up some saw horses and right next to them we set up a large fan pointing away from us next to where all of the cuts will be made. As we cut the fan sucked the dust away. It was an industrial stand up fan about 36" in diameter, but even a small box fan will help supplement a dust mask. Hardie dust has silica in it, which is harmful to one's lungs with prolonged exposure.
We bought a cheap diamond blade for our circular saw, but they also make blades specifically for hardi.
I tried the shears and it's slow. Then I bought a dust collecting saw and the cuts got much better and faster but it's still slow and it's difficult to see the blade with the dust collecting guard. To finish my project, I think I'm going to buy a sacrificial Ryobi miter saw for $100 and set up a fan as fish did.
I still need the circular saw to rip planks and I make detail cuts with my jigsaw. When I’m done, all the specialty tools are going on eBay.