How to connect two LEGO Bricks EV3 by USB using python
You can create a network connection between two EV3s using USB + ev3dev. Just enable tethering (the "Gadget" checkbox) on one of the EV3s. Connect the USB cable to the USB peripheral port on that brick (the small port next to the output ports) and the host port on the other EV3 (the USB port on the side of the brick).

Then, you could use something like RPyC to remotely control one of the EV3s. Here is an example that explains how to do this from a computer: https://sites.google.com/site/ev3python/learn_ev3_python/rpyc. So just substitute one of the EV3 bricks for the computer.