Yes, breakers are connected together when each is switching one leg of a 220-240V circuit. Eg. electric oven, hot water heater, electric dryer, or a feed to a sub-panel. All of these are typically 220-240V circuits, and require a breaker on both legs of the circuit. The breakers are physically connected to make it nigh impossible for only "half the circuit" to trip.

The more typical 110-120V circuits each use one leg from a 220-240V source, and use earth ground (from the panel, aka. "neutral") as the other leg. Therefore there is only a single live leg, and only a single breaker is required.

Cheers