I've hesitated to post this numerous times... but I keep seeing folks using switch boxes to connect a single key to multiple radios. There's an easier way with no manual switching needed. And so, here we go... The circuit shown in the jpg allows me to connect a single keyed line to multiple modern radios... The one keyed input pulls down the connected radio key lines. The diodes prevent powered off radios from keying the rest of the connected devices. I load diodes on both tip and ring allowing the splitter to work with a single keyed line as well as with a paddle. The small pcb is housed in a 3D printed case. This could be made as a simple home-brew using a few connectors and Schottky diodes. I strongly recommend this circuit only be used with modern radios switching low voltage at low current!