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.
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.
For a slow but virtually dustless way to cut siding use power.
Use your eyeware and ear protection.
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.
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.
Use a specialized blade used with the saw such as mikita hardiblade or dewalts filber cement blade.