Hercogs

Thank You for advice! I tried with ```export RMW_IMPLEMENTATION=rmw_cyclonedds_cpp```, it works then. So, basically does it mean that ```rmw_fastrtps_cpp``` implementation does not support ```SubscriptionEventCallbacks``` for multiple topic nodes?

> @Hercogs have you had a chance to try out the latest suggestion? No, not yet! I was off from work for couple of weeks! But I am planning to...

I tried with ROS2 Foxy built from source. Using `rmw_fastrtps_cpp` implementation everything worked correctly as I would expect.