Subscriber providing condition for another subscriber to subscribe a topic

edit

I'm trying to generate a node which subscribes 2 topics and uses one topic as a condition to subscribe another topic:

class odom2obj(object):
    def __init__(self):
        self._pose_sub = rospy.Subscriber('/chatter', Pose, self._check_obj)

    def _check_obj(self):
        if self._pose_sub.num_object > 0:
            # subscribe another topic # 
            # self._odom_sub = rospy.Subscriber('/chatter2', Pose, self._callback) #

    def _callback(self):

if __name__ == '__main__':
    rospy.init_node('hsr_odom2obj') 
    odom2obj()
    rospy.spin()

I was initially thinking of setting 2nd subscriber into 1st subscriber's callback function. However, I now wonder whether this is an efficient approach when the callback function of the 1st subscriber is called often. I appreciate if someone can tell any better way.