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To assign an existing engine to a layer select the channel in the "Channel View" and then activate the "Layers & Engines" tab. Select an existing engine and click on "Select engine..." and choose the engine you would like to assign to this layer. Repeat this for each layer which has no engine assigned.
One just:out instance can only be assigned to one channel (except for the engine which is assigned as a redundant engine of a channel using "N+1" redundancy) and only as either master or redundant engine, not both.