It is possible to assign the same engine to several channels as the redundant engine using the "N+1" setup. In such environments several master channels are sharing the same redundant engine (which helps to save costs). This implies that only one master engine can fail at a 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 "Flexible (N+1)" 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 "N+1" before you can assign the same just:out to several channels (using "N+1" redundancy) as the redundant engine.