First time problem

New house, A/C running. When I turned OFF the gas furnace/airhandler 120V disconnect switch two breakers tripped in the panel.

The service outlet and furnace breaker both tripped.

They are using a shared cable (red/black) for the 2 circuits and a shared box for the outlet and switch. I could actually hear the switch arc while moving it to the off position.

Could have been left this way for many years if the buyer skipped the inspection.