How to shade the area around surfaces in matplotlib

Question

I have plotted some surfaces in matplotlib and want to shade a transparent area around them. For example I want to shade the area up to defined distances (for example 0.1) above and bellow the surfaces. This is a simplified data set and code for plotting:

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
from matplotlib.ticker import (AutoMinorLocator, MultipleLocator)
%matplotlib qt5
x_s_up = np.array([[1, 1, 1], [2, 2, 2]])
y_s_up = np.array([[1, 2, 3], [1, 2, 3]])
z_s_up = np.array([[5, 5, 5], [5.1, 5.2, 5.1]])
x_s_d = np.array([[1, 1, 1], [2, 2, 2]])
y_s_d = np.array([[1, 2, 3], [1, 2, 3]])
z_s_d = np.array([[3.9, 4., 3.8], [4.1, 4.1, 4.2]])
fig = plt.figure()
ax = fig.add_subplot (111, projection="3d")
ax.plot_surface(x_s_up, y_s_up, z_s_up, color='b')
ax.plot_surface(x_s_d, y_s_d, z_s_d, color='r')
ax.set_xlabel('X'); ax.set_ylabel('Y'); ax.set_zlabel('Z')
plt.show()

In my plot I have just shown the target shade area with yellow lines for one of the surfaces. I did not colour it to avoid hiding it but in reality les alpha value of shading colur should solve this problem. In advance I very appreciate any help.