2 barrel with stock exhaust??

Hole saws are stackable, no? well thread the arbor into them both. Smaller one serves as a pilot.

Lets say you made a hole 1-1/2”,realized its too small. How do you make it larger when pilot for hole saw is gone. Use the 1-1/2” as the pilot and thread it into the one you need. Works slick.
I wasn't aware that you could do that. I'm embarrassed to admit I've never ran into a project yet that required me to use a hole saw. Thanks for the tip! I'll remember that one for when the time rolls around.