In addition to your primary second stage regulator all scuba regulators should include a safe second regulator or Octo. This alternate air source comes into play in the unlikely event of the you need to share air with your buddy or something happens to your primary.
Your Alternate Air Sources come in a variety of styles. The most common are your basic octo which is a second hose connected to the first stage, usually brightly colored so it is easy to find. The other is integrated into the inflator of your BCD, which allows the scuba diver to be more streamlined by allowing the alternate air source(octo) to be combined with the inflator of the BCD thus eliminated an extra hose hanging from your body. This is the preferred configuration for our Instructors.
Another popular option is a complete Redundat Breathing System like the Spare Air.
Find your Scuba Octopus at Divers Direct. We have top name brand Octos including Aqua Lung ABS, Mares Air Control integrated, Atomic SS1, and more.