Concurrent service clients in ROS 2

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I want to run 2 client to service Sum1 and Sum2 at the same time. I had tried MultiThreadedExecutor but they are running sequentially. Below is my code:

class Test_client(Node):
    def __init__(self,sv):
        super().__init__(sv)
        self.name = sv
        self.group = ReentrantCallbackGroup()
        # timer_cb_group = client_cb_group
        self.cli = self.create_client(SumInt,sv,callback_group= self.group)
        self.req = SumInt.Request()
        self.call_timer = self.create_timer(1, self.get_result(), callback_group=self.group)

    def send_req(self,a,b):
        # b =5
        # a = 8
        start = time.time()
        print("ok"+ str(time.time()))
        self.req.a = a
        self.req.b = b
        self.future = self.cli.call_async(self.req)
        self.executor.spin_once_until_future_complete(self, self.future)
        print(self.future.result())
        return self.future.result()

def main():
    rclpy.init()
    test_client =Test_client('Sum1')
    test_client2 = Test_client('Sum2')
    executor = MultiThreadedExecutor(3)
    try: 
        executor.add_node(test_client)
        executor.add_node(test_client2)
        f1 = test_client.send_req(2,3)
        f2 = test_client2.send_req(5,6)
        try: 
            executor.spin()
        finally:
            test_client.destroy_node()
            test_client2.destroy_node()
            executor.shutdown()
    finally:
        rclpy.shutdown()


if __name__ == '__main__':
    main()