I would like to know how to model a building being served by the following system.
System 1 is a ventilation only system that conditions the OA to 72 degree F.
System 2 is a cooling only VAV air handling system that cools the air to 55 degree F (This system has an economizer mode).
The two ducted air streams supplied by the above System 1&2 would only come into contact right before being discharged off the floor diffusers.

Ventilation air can also be supplied by System 2 (cooling only unit) when they run in economizer mode.
In addition, a floor mounted 4-pipe fan coil unit system is used for the parameter heating and cooling.

your suggestions and inputs would be greatly appreciated.

Guo Hong Ho