You can create up to four linked EV3s (a "Master" and three layers of "Slaves"). Refer tutorial attached for instructions.

To access sensor values on a sensor connected to a Slave, you need to use the "Sensor Port" constant.  It is not possible to do this by simply "right-click > create constant" on the Read Sensor function.  Below is an example configuration, program, and indication of where the Sensor Port constant is located (IO >> Complete >> Sensors >> Sensor Port).  This will then use the configuration you previously specified.