Sending more than two images

import cv2
import numpy as np

def stitch_jpgs(jpg_a, jpg_b):
    image_a = cv2.imdecode(np.frombuffer(jpg_a, dtype=np.uint8), cv2.IMREAD_COLOR)
    image_b = cv2.imdecode(np.frombuffer(jpg_b, dtype=np.uint8), cv2.IMREAD_COLOR)
    image_stitched = np.concatenate([image_a, image_b])
    return cv2.imencode(".jpg", img)[1].tostring()

import rospy
from sensor_msgs.msg import CompressedImage
import cv2
import numpy as np

TOPIC_A = "/path/fisheye_camera_front/image_raw/compressed"
TOPIC_B = "/path/fisheye_camera_back/image_raw/compressed"
STITCH_TOPIC = "/path/fisheye_camera/stitched/compressed"

def ros_jpg_to_cv2(msg):
    return cv2.imdecode(np.frombuffer(msg.data, dtype=np.uint8), cv2.IMREAD_COLOR)

def cv2_to_ros_jpg(img):
    cv_encode = cv2.imencode(".jpg", img)[1]
    compressed_image = np.array(cv_encode).tostring()
    compressed_image_msg = CompressedImage()
    compressed_image_msg.format = "jpg"
    compressed_image_msg.data = compressed_image
    return compressed_image_msg

class Stitcher:
    def __init__(self):
        self.topic_a_img = None
        self.topic_b_img = None
        self.pub = rospy.Publisher(STITCH_TOPIC, CompressedImage, queue_size=10)
        rospy.Subscriber(TOPIC_A, CompressedImage, self.topic_a_callback)
        rospy.Subscriber(TOPIC_B, CompressedImage, self.topic_b_callback)

    def topic_b_callback(self, msg):
        self.topic_b_img = ros_jpg_to_cv2(msg)

    # When topic A is received, publish the full image
    def topic_a_callback(self, msg):
        self.topic_a_img = ros_jpg_to_cv2(msg)
        if self.topic_b_img is not None:
            stitched = np.concatenate([self.topic_a_img, self.topic_b_img])
            self.pub.publish(cv2_to_ros_jpg(stitched))

if __name__ == "__main__":
    rospy.init_node("stitcher")
    Stitcher()
    rospy.spin()