Just my opinion but I don't believe it will function properly unless they've manufactured a 2-pole arc fault breaker and you're connecting both branch circuits. My reasoning is this, if you stop and think about it the common neutral is not landed on the ground bar in the breaker panel. The pigtail of the AF breaker is connected to the bar and the common neutral is connected to the AF breaker. That means only one of the branch circuit phase conductors would be connected to the AF breaker. What do you do with the other ? Connect it to another AF breaker ? You can't common up the neutrals. Only one set of wires (hot & neutral) can be monitored through an AF breaker unless, as I mentioned, they've made a 2-pole AF breaker.