I wanted to reply/add to your point of information post Pete.

"Point of Information-

The Yale Bicentric cylinder was an early answer to avoiding the problem of key interchange for masterkeyed locks. Most pin tumbler masterkeyed locks use a "split-pin" system and as a result many unintended keys may operate the lock. As an example, a 6-pin cylinder with a single master key disc in all six chambers could be operated by 64 different keys! The Bicentric cylinder has two plugs, neither of which is masterkeyed. One, normally the upper plug, is controlled by the master key while the other plug, usually the lower one, is controlled by the change key. The lock can be operated by the use of either key in its own plug. Thus it is not dual custody in the sense that both keys would be needed to open the lock like a safety deposit box."
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While the majority of these locks I see are opened by either key, they did make a dual custody configuration with 'guard' and 'change' cylinders. I've included a couple pictures of one I have. The guard key must first be turned before the change key will open, neither will open the lock by itself.
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