Understanding neural network probability
What you have declared here is actually a 2-class multi-label classifier. It is trying to learn the probability of being each class independently where they are not mutually exclusive. 

While that is a valid classifier problem to solve, I'm guessing what you mean is that you have an exclusive 2-class problem: it's either positive, or negative. In that case, you want the final layer to be Dense(1, activation='sigmoid')