For cases where your images happen to be the same size (which is a common case for displaying image processing results), you can use numpy’s concatenate to simplify your code.
To stack vertically (img1 over img2):
vis = np.concatenate((img1, img2), axis=0)
To stack horizontally (img1 to the left of img2):
vis = np.concatenate((img1, img2), axis=1)
To verify:
import cv2
import numpy as np
img1 = cv2.imread('img1.png')
img2 = cv2.imread('img2.png')
vis = np.concatenate((img1, img2), axis=1)
cv2.imwrite('out.png', vis)
The out.png image will contain img1 on the left and img2 on the right.