Your question is too broad. What kind of dual drainage system you want to mode – for example, do you really want to model each inlet opening as a weir/orifice or just a simply manhole without flow restriction? Your budget and time?
Modeling a dual drainage system in EPA SWMM is very time consuming (still doable). You may want to use SWMM commercial version instead if they have some built-in features to model street inlets and gutter.
