electical service box

I have a Bryant electrical service box that is full. It is rated 100 amps and is 35 years old. I bought two twin breakers so I can add two more circuits. The diagram on the back of the cover panel doesn`t indicate which of the slots to use for a twin breaker. Can anyone help? I am not worried about overloading as our home is occupied by only myself and my wife and we are not heavy electrical users.
Location: USA
------------------------------------- JDS
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