Add some plugin with a gain or volume control to the track you want to control. If you’ve already got plugins on the track, pick one. Otherwise you can use the JS Volume Smoother.
Create a send from the track you want to control the other and set it to go to channels 3/4 on the track you want controlled. Wiggle the plugin gain/volume control that you picked above. Click the Param button up toward the top of the plugin window and go to Parameter Modulation. Click the check box next to Audio Control Signal. Choose Track Audio Channels 3/4. Adjust the other settings to taste.
https://m.youtube.com/watch?v=c8IvOCbUo2U
It’s sometimes a good idea to put another plugin after that plug to block the extra channels from passing up to any parent tracks which might have their own sidechain thing happening. Any plugin that doesnt make noise of its own, but preferably also doesn’t use much CPU. ReaEQ is a good choice, but there are plenty of options. Click the 2 in/2 out button at the top of that plugin. Uncheck both input boxes and make sure that the only output boxes checked are for channels 3/4. This way it takes no input, so outputs silence, and overwrites channels 3/4 with nothing.