If I’m not mistaken, there are actually no direct support for linking two nodes by a wire.
However, you should be able to connect both nodes to a computer, and run a python script to flood all messages across both nodes. This might not be tested yet, but should, or rather might, work. Moreover, nodes might even learn the whole network (add nodes on different MHz to their NodeDB), as the packets flow trough LoRa.
@geeksville should be able to help you with the python code, if you run into any problems. 
Examples of python: Anyone In Italy?
Some info on setting up repeaters: Solar relays - best practice?