We want to register base2 (secondary) to base1 (primary). This step is not necessary but advisable so that the two bases do not interfere with each other. To do this, the cover of the DX200 has to come off and dipswitch 4 must be set to "on" or "up" on the transceiver board of base2. Once this is done and power is turned on for base 2, it will come up with 2 lines on the display, letting you know that it is a secondary base waiting to be registered to a primary base. Press the registration button on the primary. Next press the registration button on the secondary base until a "2" comes on the display and wait. When the secondary base successfully registers, a single line will come on the display of the secondary base. Next, connect the two bases, AUX IN (base1) to AUX OUT (base2) and AUX OUT (base1) to AUX IN (base2). Terminate one of the bases. Lastly, press the ISO+ button on the front of the base. This will allow all the headsets to talk to each other on the ISO channel (button) not the IC button.