It is possible to assign an engine as the redundant engine using the "2N" setup. In such environments each channel has a master engine and a dedicated redundant engine assigned to it, where both engines are playing the same content at the same time.
To assign an existing engine as a redundant engine to a layer you must activate the "Redundancy" checkbox under the "Advanced" tab first and choose "Full (2N)" after that. Select the channel and then activate the "Layers & Engines" tab. Click on "Select engine..." under the "Redundant just:out Engine" tab and choose the engine you would like to assign to this layer. Repeat this for each layer which has no engine assigned.
Because you selected "2N" before, you can assign the same just:out only once to a channel.