Use a specialized blade used with the saw such as mikita hardiblade or dewalts filber cement blade.
Cutting cement board siding.
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.
When cutting fiber cement siding with circular saw use a blade that has only four or six teeth.
Typically siding shears use air pressure to power them and they shear or cut the siding like a pair of scissors.
If any cutting needs to be done to allow for the added flashing than it can be done on the workbench with a pair of siding shears zero dust.
The cleanest way to cut fiber cement siding is by using siding shears.
These are specialized to reduce the dust caused from cutting these planks.
For a slow but virtually dustless way to cut siding use power.
You have to make the cuts very slowly.
Use your eyeware and ear protection.