Typically siding shears use air pressure to power them and they shear or cut the siding like a pair of scissors.
Cutting cement board siding.
You have to make the cuts very slowly.
The advantage of using the circular saw is that you can cut more than one board at a time.
To keep dust to a minimum use a circular saw that s equipped with either a dust collection bag or electric fan that.
When cutting fiber cement siding with circular saw use a blade that has only four or six teeth.
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.
These are specialized to reduce the dust caused from cutting these planks.
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.
Use a specialized blade used with the saw such as mikita hardiblade or dewalts filber cement blade.
I recommend the turboshear tsf1a fiber cement pnuematic if you re looking to buy a pneumatic siding shear.