Tether communication question with another tether board set

We have another companion computer, jeston in ROV for stereo-camera, image sonar and other sensors processing. Since original one twist-pair (two wires) only have 10MB/s, we plan another tether board set connecting another twist-pair for data transmission for jetson.

During the test, Jetson is connect with ROV-side tether board with switch, tether board connected with another twist-pair from tether cable. I didn’t connect the top-side tether board at all. I just connect the original tether board with my laptop, after I change the jetso’s IP to 192.168.2.3. Then I can ping it. How is that come, because jetson should not connected with my laptop.

I am not a Ethernet or communication expert. Thanks for your suggestion.