VAV rooftop unit + VAV DOAS in the same zone

edit

You could try with the ZoneHVAC:OutdoorAir Unit as you mentioned but it looks a bit like a black box (heating, cooling, and heat recovery seem to be just built in) so I'm not sure you could do the humidity control that you want.

Alternatively, you could try to model the DOAS and RTU together as one E+ Airloop. You could put the DOAS components on the outdoor air path (between the OA node and the OA Mixer). If the DOAS serves multiple zones you won't be able to capture part load performance exactly (the many small DX coils representing the central DX coil will likely all be running at different part load ratios; same for fans), but it should be reasonably close. If the DOAS only serves that zone, even better! Note that I have done this successfully in the past for DOAS heating and cooling but not for DOAS fan power. In my case, it was a constant volume system so I represented fan power using a separate process load (and ignored fan heat), but since you have a variable speed fan, you'll want to model the fan explicitly. My understanding is that there is nothing stopping you from putting a variable speed fan on the OA path. I would try it and see what you get. You should be able to apply DCV or whatever other control that you have that results in variable OA and get the right flow rate (and associated fan power) for the DOAS fan.