The valve shutdown drill should be in your skill set if you dive with two regulator connected to one breathing source (manifolded twinset or single tank with a double valve). The problem you want to solve is the leakage of gas from one of your regulators. There are plenty of different ways to do it, but here I will only write about one of them for a manifolded twinset with an isolator valve.
Necessary ability:
You are able to reach the valves (isolator, right cylinder, left cylinder).
Drill procedure:
1. Close the isolator valve
2. Switch to the left regulator
3. Close the right cylinder valve
4. Purge right regulator carefully
5. Open right cylinder valve
6. Switch to right regulator
7. Close left cylinder valve
8. Purge left regulator carefully
9. Open the left cylinder valve
10. Open the isolator valve
Real procedure:
1. Close the isolator valve
2. Switch to the left regulator
3. Close the right cylinder valve
4. Purge right regulator carefully
–> No bubbles afterwards? Then open isolator valve and return to the surface.
5. Open right cylinder valve
6. Switch to right regulator
7. Close left cylinder valve
8. Purge left regulator carefully
–> No bubbles afterwards? Then open isolator valve and return to the surface.
9. Open the left cylinder valve
–> Non of your regulators have a leak, but maby a valve. Return to the surface.
Always try to trigger your buddy as soon as possible with your unused hand either via torch or stick.