Yes, it is possible to connect your two MikroTik RB2011 to a central MikroTik to handle the traffic of your 18 Internet lines from a single LAN port.

This process involves setting up a more advanced routing and load balancing scenario, using what is commonly called in MikroTik as “recursive load balancing” or simply a layered load balancing approach.

Here we explain how you could configure it:

Configuration of the MikroTik RB2011

1. Initial setup: Make sure each MikroTik RB2011 is correctly configured to balance the 9 Internet lines it has connected. This can be done using the PCC (Per Connection Classifier) ​​load balancing method, which distributes sessions evenly across available lines.
2. Connection to MikroTik Central: Each RB2011 should have one output configured that connects directly to the central MikroTik. This output can be configured as an uplink interface and carry balanced traffic from all 9 lines.

MikroTik Central Configuration

1. Team selection: The core equipment should be a more robust model if it is expected to handle the aggregate traffic of 18 Internet lines, something like a MikroTik CCR (Cloud Core Router) might be appropriate due to its greater processing capacity and throughput.
2. Routing and balancing configuration: On the central MikroTik, you will need to configure the input routes of each RB2011 to continue load balancing. Here you can also use PCC or any other load balancing method you deem appropriate to handle multiple uplinks.
3. Single output for LAN: Configure one of the central MikroTik's LAN interfaces to distribute balanced traffic to your local network. This involves ensuring that traffic coming through the interfaces connected to the RB2011 is distributed correctly through this single LAN output.

Additional considerations

• High Availability and Failover: Consider implementing VRRP (Virtual Router Redundancy Protocol) or a similar configuration to provide redundancy and failover in case one of the RB2011s fails.
• Monitoring and Administration: Use tools like The Dude by MikroTik to monitor connection status and network performance. This is crucial to detect bottlenecks or failures in the system.
• Safety: Don't forget to configure firewalls and other security measures to protect your network against unauthorized access and external attacks.

If you are unsure how to perform any of these configurations, I recommend consulting a MikroTik networking expert or looking for detailed MikroTik-specific guides for each of these steps.

Also, you can consider taking official MikroTik courses to deepen your knowledge of advanced network configurations.