JValdez, it's not a MGE command... what i call double lock key are a couple of tech interdicting each other, used as prerequisite for tribe units; so it's placed in rules.txt.
Example:
This is the shape i call simple key...
In an hypotetic Greek Bactriane scenario
all greek units --- prerequisite, Greeks, obsolete, Indians
all indian units --- prerequisite, Indians, obsolete, Greeks
Of course, this way it's more easy to take forbidden techs by espionage or diplomacy..
This is the double lock key shape:
all greek units, prerequisite, Greeks, obsolete, non Greeks
all Indian units, prerequisite, Indians, obsolete, non Indians
where greeks have non Indians and Indians have non Greeks techs. This way is more difficult to exchange or trade the forbidden tech.