The objects listed in the AirLoopHVAC:OutdoorAirSystem:EquipmentList, for example:
AirLoopHVAC:OutdoorAirSystem:EquipmentList,
VAV_1_OA_Equipment, !- Name
HeatExchanger:AirToAir:SensibleAndLatent, !- Component 1 Object Type
VAV_1 OA Heat Recovery, !- Component 1 Name
OutdoorAir:Mixer, !- Component 2 Object Type
VAV_1_OAMixing Box; !- Component 2 Name
are listed in simulation order. For example here the HX would be simulated before the outdoor air mixer so that the OA mixer would know the correct inlet air temperature. The only way I can think to do this would be to use mixers and splitters to create and gather the multiple air streams. The problem is that these mixers and splitters will not be simulated since they would not be listed anywhere in the input file. If a mixer/splitter were allowed in the AirLoopHVAC:OutdoorAirSystem:EquipmentList then that object would be simulated/modeled. Only OA mixers, fans, coils, HXs, VRF TUs, UnitarySystem, CoilSystem, evap coolers, humidifiers, dehumidifiers, and solar collectors are allowed in the OA system equipment list. For now I think you are limited to modeling these object in series or as a single BIPVT object with properties similar to those of the objects you intend to model.