With the 2, you are doing it less frequently and still only emptying/refilling one at a time. Position 1 moves to position 2. Position 2 is emptied/refilled and takes the place of position 1. Squeezes more out of the canister by letting it scrub whatever the first one didn't.
I see. However still requires an action to refill and switch positions. Wondering if a single larger can would last longer and require less touching. =) Lazy reefer!