How to model 100% Outdoor Air AHU in Series with Additional AHU

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I am trying to model a system in which an Air Handler (call it AHU-1) conditions 100% outdoor air and then passes the air to a second AHU (AHU-2) where it mixes with return air, passes through a cooling coil, and is then discharged to various thermal zones. Just to reiterate, AHU-1 does not directly supply any air to the zones.

Thus far, I've been using OpenStudio to do my modeling, but I'm getting the feeling that I'll need to go into the IDF Editor with which I have almost no experience.

There are additional challenges in modeling the system, but I think I'll save those for another time.. Any help is greatly appreciated!