A popular post about 'stacking' can be found here: <https://stats.stackexchange.com/questions/185400/ensemble-learning-why-is-model-stacking-effective?noredirect=1&lq=1>. The author said that 'stacking' can easily improve the performance with respect to the individual base learners. But I found it not that easy, at least in my project.

Many posts said the key for stacking to work consists in many different base learners are used. From my understanding, it is like the Fourier series in which a complex function can be approximated by a series of triangular base functions. It required that the base triangular fucntions are different in frequency, in other words, they represent different characteristics of the objective functions. In model stacking, we also required the base learners to be different that can capture different features of the underlying complex functions.

Actually, I calculated the simple Pearson correlation of the two methods A and B in the question, and found the correlation coefficient to be only 0.2. Another reason might be the size of sample is too small.