There is no sidechain implementation in Pyramix.
But there could be a SideChain/Ducking workaround by using the db audioware VST, see the screenshot attached where they have a Universal Sidechain link notion and no need to mix the inputs to obtain a Ducking effect,
Details here with video, where you can download the plugin demo.
There could be a different workaround as you can see bellow, you can create a multichannel strip and apply it from there. For example with Flux plugin which must receive both medias sources simultaneously in order to make a Process/Sidechain relation.
Another example of using the Side Chain that is build in the Flux PureComp.
In this case again the synth on 1-2 will playback and duck when the dialog on channels 3-4 enters.
Refer to Side Chain Internal routing to achive this.
Flux tutorial below on the feature.