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The Apogee Duet is a 2 input and 2 output audio interface... this means you only have the option to use the Output 1-2, which is why there is no Output 3-4 available from within Logic.
There are two real solutions... either get an interface with more outputs, or try creating an aggregate device.
An Aggregate device combines the inputs and outputs of two audio interfaces. This will give you a greater number of inputs and outputs to play with. As your Mac has a built-in audio interface, if you create an aggregate device combining the Duet and the Built-in you will in theory have 4 outputs and could use the headphone port on your Mac as the Headphone Mix output.
Check this document out for how to create an aggregate device: