How to subcribe 2 image topics with 1 node in same time ?

edit

Although, I'll be honest I don't recognise the error message you're getting I can see a bug in your code.

self.image_sub = rospy.Subscriber("/kinect/rgb",Image,self.callback_rgb)      #Case 1
self.image_sub = rospy.Subscriber("/kinect/depth",Image,self.callback_depth)  #Case 2

In these two lines you're writing the subscriber objects to the same variable self.image_sub so the second one should destroy the first. Try changing the these to two different identifiers, and see if the error continues.

Yes, the second one would destroy the first one. But looks like you want to have both the information pieces accessible and process them at the same time. If that's the case, you need to use TimeSynchronizer from message_filters package. It allows you to have a single callback function for various topics. In your case, it would be:

#!/usr/bin/env python
import sys
import message_filters ### ADD THIS
import rospy
import numpy as np
import math
from std_msgs.msg import String
from sensor_msgs.msg import Image
from cv_bridge import CvBridge, CvBridgeError
import cv2

#------Start of Class
class image_converter:
    def __init__(self):
        self.bridge = CvBridge()
        self.image_rgb_sub = message_filters.Subscriber("/kinect/rgb",Image)      #Case 1
        self.image_depth_sub = message_filters.Subscriber("/kinect/depth",Image)  #Case 2

    def callback(self, rgb,depth):
        cv_image_rgb = self.bridge.imgmsg_to_cv2(rgb, "bgr8")
        cv2.imshow("RGB_Sub", cv_image_rgb)
        cv_image_depth = self.bridge.imgmsg_to_cv2(depth, "bgr8")
        cv2.imshow("Depth_Sub", cv_image_depth)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            rospy.signal_shutdown('Quit')
            cv2.destroyAllWindows()

def main():
    img_cvt = image_converter()
    rospy.init_node('image_converter', anonymous=True)
    try:
        ts = message_filters.ApproximateTimeSynchronizer([img_cvt.image_rgb_sub,img_cvt.image_depth_sub],10,0.1)
        ts.registerCallback(callback)
        rospy.spin()
    except KeyboardInterrupt:
        print("Shutting down")

if __name__ == '__main__':
    main()

More info can be found on this documentation.

Peace!