Plotting a 2D iso-probability curve
Since you are using NumPy, you should deal with grids the NumPy way instead of performing list comprehensions.

def probability_diff(x0, x1):
    probs = library_function(x0, x1)
    return probs[0] - probs[1]

results = np.vectorize(probability_diff)(X0[:, np.newaxis], X1)


I wouldn't try to get too clever with the lambdas.

Also, since the grid happens to be a square, you could use X0 again instead of defining X1.