How to know when tandem breakers can be used

What is the maximum number of circuit breakers allowed in an two breakers in one slot That diagram denotes where it's permissible to install tandem breakers Unless it's a GE two slot double breaker with 2 individual breakershandles that go on one bus for 2 top slot on each side of the panel will only accept 1 breakers All the

two slot shelving Please see comment below for information on how to combine 2 Double Pull Breakers to free