The valve drill is a key skill when diving a twinset and something that is fully taught as part of the GUE fundamentals course. The purpose of the valve drill is to make sure your valves are in the right position (which is fully open if all are working properly) and that you can open and close them while in control.  Make sure you have the full attention of your team when manipulaing valves in the water.

 Do not underestimate the importance of a quick and efficient shutdown if a problem occurs. Those who struggle to shut down, yet rely on their twinset for redundancy, are fooling themselves, and those they dive with, that they are in some way safer than if they dive with a single tank. At 60 metres a low pressure hose failure will empty a twinset in around 20 seconds. It is a skill worth refining to the point that it is quick, effortless and instinctive. So how? 

The basic doubles valve drill involves closing and opening all three posts on a twin set starting with the right post, then the isolator, then the left post. I've put tips on how to reach your valves in this section here also - but here is a quick video showing the drill.