Redundancy is a key concept involved in technical diving. Multiple cylinders not only enable the diver to carry the increased required amount of gas, but also to isolate any equipment malfunctions while saving reserves. Sidemount diving is no different. As tanks are used separately and not connected by a manifold, isolating the malfunction allows conservation of gas in the reserve tank. This requires a higher awareness in gas usage, but further increases the ease of isolating these problems because it can be seen visual because if their position on the flank rather then the back on the diver as tanks are used separately and not connected.
The placement of the first stages under the arm also help to easily recognize the problem and fix it accordingly. While worn on the back, the hardest part of a shutdown situation is finding and recognizing in the problem. With both first stages in plain view, find the problem while diving in a sidemount configuration becomes easier and fixing the problem then simply becomes shutting down the problem cylinder and switching to the reserve if needed.
Photographers (Team Blue Immersion Fx) and other who find themselves diving solo (if professionally educated and holding solo diver certification) find this reserve system vital. Speaking from experience as videographer for many years, occasions arise where you may find yourself by yourself. In normal recreational gear, this could be particular dangerous if problems were to arise. When properly trained with the knowledge of how to use a sidemount configuration. Dives to gather stock footage or video can not only last longer, but be carried out in a much safer manor, while still enjoying the comfort and freedom of a sidemount setup.