Cutting Wood Siding in Place
Set the depth of the blade on your circular saw to be just deep enough to cut through the siding. Err on the side of slightly too shallow, and come back with a chisel, utility knife, or handsaw to finish.

If your circular saw's base plate is not long enough to sit across two planks of siding, you could fashion a jig to hold it. The jig could have a long base so it sat across 3 or 4 planks of siding. The jig just needs to be a long piece of plywood with simple stops on the 4 sides; the stops would hold the saw's base plate in place.