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You have assessed what OpenStudio is doing correctly. Those inner nodes of the unitary system do not get created until the idf file is created. If you really want to know, that point is right here in the osm->idf translator. You could use this information to predict the names of the generated nodes so that you can add a setpoint manager later, perhaps with an IDF Measure.

I suggest instead that if you are using the setpoint mode of AirLoopHVACUnitarySystem, just wrap a single component in each unitary. If you have a heating coil and a cooling coil, use one unitary for each. I have done this myself in the CBECC-COM -> OS Translator. E+ will have no issue with multiple unitary systems on a single AirLoopHVAC. You are just using the unitary as a coil wrapper in this case to allow the component you want on the air loop.

One of the main value propositions of OpenStudio when it comes to HVAC is that it manages the (complicated) connectivity of things for you. Unfortunately the fancy mechanisms OS uses to connect HVAC components to each other do not extend to the inside of compound HVAC components. Hopefully we will have time to improve this in the future. When that happens the inner nodes will be instantiated and connected together in the osm so that you can reference them and attach things to them.