Okay, that would explain why you are seeing only one node in the Monitoring UI. The UUID uniquely identifies a node so two nodes with the same UUID will be seen as one.
The solution is to simply stop one of your two Logstash nodes, delete the data/uuid file for that node and then restart the node. Logstash will recreate the data/uuid file automatically upon restart, and store a new, unique UUID in it. Then, if you look in the Monitoring UI, you should start seeing two Logstash instances.