It normally just goes from how many people you book the room for
ie -
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They don't have to be named, and they don't have to attend check in. (For all they know, they're coming later, they're parking the car, whatever - you don't even need to ask for two keys).
That should give you the double credit automatically (95% of the time). If they only give you one when you check out you can simply ask for the other credit.
I've never had a problem with it; it's a bit stupid they don't just give you two credits regardless as you're paying the same price.