I have two Tasmota enabled devices that I want to turn on/off at the same time. In my case I have a wall switch that powers the main lights, and a wall socket that powers some corner lights and a radio. Using Tasmota's rule system this is not too complicated. We need tell each Tasmota device that whenever the state of it's relay changes (for whatever reason: button, timer, etc.) it needs to tell the other Tasmota device to set it's relay to the matching state. We can use the WebSend feature built in to Tasmota to send commands between two devices.

On device #1

RULE1 on POWER1#State do WebSend [192.168.1.33] POWER1 %value% ENDON

On device #2

            RULE1 on POWER1#State do WebSend [192.168.1.34] POWER1 %value% ENDON

This can also be done using MQTT pretty simply, but I wanted to see if I could do it with vanilla Tasmota. Doing it direct with WebSend is pretty simple and saves having to run an MQTT broker if you just have two simple devices.