Your question is not phrased as I would expect and would benefit from some clarification to be sure that I am reading it correctly. As I understand the question you are asking:
"If I have a single event and can choose between 2 decision factors (A and B) to determine the probability of the event, where A and B have probabilities of being correct of P(A) and P(B), what is the total probability of making the correct decision if I use both factors".
Assuming this to be the case and there is only 1 event but there are 2 unrelated decision factors it is additive: P(A)+P(B).
It would be P(A)*P(B) if we were considering the probability of 2 independent events occurring each with probabilities P(A) and P(B) but, judging by your question it is the former and not the latter case in which you are interested.
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