Hawkins is right ... however, you still want trigger B to function as a normal trigger, so in trigger B you actually need:-
Condition: this and trigger1
In other words trigger B will fire only when the Activation box is satisfied (east present or whatever, that's the "this" part of the condition line) and also trigger A has not been fired (the variable trigger1 is still true, having been set to true in your init.sqs file or the init field of a unit)