Hey @yunwool
Just to be clear, consider only your first drone, if you run export ROS_DOMAIN_ID=1, then run ros2 topic list, you aren't seeing any topics right?
After you set the ROS_DOMAIN_ID of a drone to 1, to get the PX4 topics to show up, on QGC > Analyze Tools > MAVLink Console, run the following commands:
param set XRCE_DDS_DOM_ID 1
param save
Then power cycle the drone, give it like 15s after boot up, then the topics should come up directly when running ros2 topic list on that drone.