can a node listen to two other nodes?

edit

Hey forum members, greetings to you all!

I am new to ROS, I was wondering if it is possible to create a node which will listen to a topic where two other nodes are publishing information.

Let's say I have even_number_node that generates even numbers and odd_number_node that generates odd numbers, they both are publishing let's say std_msgs::UInt64 data to a mathTopic, this third node adder_node will subscribe to the mathTopic and will get odd and even numbers generated by previous nodes, how can I identify which data is sent from which node and find sum of them?

I actually attempted to make it work based on the talker-listener nodes provided in ROS tutorials but messed everything up. Yes, I can create such a thing with service-client node but I want to break it down to be able to understand the details.

I know this sounds pretty much a dumb questions to experts, so thanks for the help!

Update1: Got it to work! Please see update2 below!

I was playing with the code and now it says: build system error and takes me to the following line: message(FATAL_ERROR "\nAssertion failed: file '${FILENAME}' does not exist. Message: ${MESSAGE}\n")

// CMakeLists.txt includes:

add_message_files(
   FILES
   twoData.msg)

// twoData.msg file includes:

    std_msgs/UInt64 data1
    std_msgs/UInt64 data2

evenNumberGenerator node, oddNumberGenerator node is the same with minor changes described below in comments:

#include "ros/ros.h"
#include "std_msgs/UInt64.h"
#include "mathematics/msg/twoData.h"

int main(int argc, char **argv)
{
    // initialize evenNumberGenerator
    ros::init(argc, argv, "evenNumberGenerator"); //oddNumberGenerator node for the other node

    ros::NodeHandle nodeObject;

    // publish to mathTopic
    ros::Publisher publisherObject = nodeObject.advertise<twoData::UInt64>("mathTopic", 1);

    ros::Rate loop_rate(1);

    int i = 1;

    while(ros::ok())
    {
        twoData::UInt64 number;

        while(ros::ok())
        {
            number.data2 = 2*i; //this is number.data1 = 2*i-1; for oddNumberGenerator node
            i++;
            ROS_INFO("%lli", number.data2); // number.data1 for the other node

            loop_rate.sleep();

            publisherObject.publish(number);
        }

    }

    return 0;
}

Update2: I got it to work. As @ahendrix said each node sends a complete message, not part of it, like:

node1 sends:

data1: 1 # odd numbers
data2: 0
---------------
data1: 3
data2: 0
--------------
...

and node2 sends:

data1: 0
data2: 2 # even numbers
---------------
data1: 0
data2: 4
--------------
...

and the third node which is supposed to listen to both of them at the same topic returns:

I believe oddNumberGenerator says: 1 # data from oddNumberGenerator
I believe evenNumberGenerator says: 0 
------------------------------------------------------------
I believe oddNumberGenerator says: 0 # data from evenNumberGenerator
I believe evenNumberGenerator says: 2 
------------------------------------------------------------
I believe oddNumberGenerator says: 3 # data from oddNumberGenerator
I believe evenNumberGenerator says: 0
------------------------------------------------------------
I believe oddNumberGenerator says: 0 # data from evenNumberGenerator
I believe evenNumberGenerator says: 4
------------------------------------------------------------
I believe oddNumberGenerator says: 5 # data from oddNumberGenerator
I believe evenNumberGenerator says: 0
------------------------------------------------------------
I believe oddNumberGenerator says: 0 # data from evenNumberGenerator
I believe evenNumberGenerator says: 6
------------------------------------------------------------
...

As expected, so I decided to go with @ahendrix's advise on Time Synchronizer and modified my code as it is below:

#include "ros/ros.h"
#include "mathematics/TwoData.h"
#include "message_filters ...