To keep dust to a minimum use a circular saw that s equipped with either a dust collection bag or electric fan that.
Cutting cement board siding.
You have to make the cuts very slowly.
Use a specialized blade used with the saw such as mikita hardiblade or dewalts filber cement blade.
For a slow but virtually dustless way to cut siding use power.
Typically siding shears use air pressure to power them and they shear or cut the siding like a pair of scissors.
When cutting fiber cement siding with circular saw use a blade that has only four or six teeth.
These are specialized to reduce the dust caused from cutting these planks.
One way to eliminate cutting or at least keep it to a minimum would be to remove the entire piece above the window flash the window and then replace the siding.
The cleanest way to cut fiber cement siding is by using siding shears.
I recommend the turboshear tsf1a fiber cement pnuematic if you re looking to buy a pneumatic siding shear.