How can I subscribe to 2 different topics in a synchronised manner in ROS2 ?

edit

I have 3 different nodes : Node2 subscribes to two different topics of same types published by node 1 and node3. Then it does some calculations on these subscribed topics and the publishes to a new topic which is then subscribed by node 3. So basically I want to test looping calculations.

Here is my class definition for node2 which is subscribing to 1 topic and publishing to 1 :

    class HeatConduction1 : public rclcpp::Node
{
public:
  explicit HeatConduction1(const std::string & topic_subscriber1,const std::string & topic_publisher)
  : Node("heatConduction1")
  {


    auto callback =
      [this](const std_msgs::msg::Float32::SharedPtr msg_) -> void
      {

    heatflow = calculateHeatFlow(heatflow,tempPM1,msg_->data);
    tempPM1 = calculateTempPM1(tempPM1, msg_->data,heatflow);



    RCLCPP_INFO(this->get_logger(), "I heard from source node the source heatflow: [%f]", msg_->data)
    //RCLCPP_INFO(this->get_logger(), "I heard from heatconduction2 node as the tempPM2: [%f]", msg_2->data)


    pub_msg = std::make_shared<std_msgs::msg::Float32>();

    pub_msg->data = tempPM1;
    RCLCPP_INFO(this->get_logger(), "Publishing: '%f'", pub_msg->data)
    pub_->publish(pub_msg);
      };


    sub_ = create_subscription<std_msgs::msg::Float32>(topic_subscriber1, callback);
   // sub_1 = create_subscription<std_msgs::msg::Float32>(topic_subscriber2, callback);

    pub_ = this->create_publisher<std_msgs::msg::Float32>(topic_publisher, rmw_qos_profile_default);

   }

Now, how can i modify this code to add another subscriber. Can I pass another argument in the constructor naming const std::string & topic_subscriber2 and then create another shared pointer as sub_1 and use a callback command as sub_1 = create_subscription<std_msgs::msg::Float32>(topic_subscriber2, callback); But this is not working as it gives a compilation error.

Or should i have another callback function in the constructor and call it seperately ? but in this case how would the callbacks be synchronised ?

Any helps would be appreciated.