DDSWMM is based on the dual drainage principle. It views urban drainage systems as two distinct, but interconnected networks: a) a minor or convenience system, formed by the underground sewers, and b) a major system, which is formed by the streets. Storm inlets, which intercept surface runoff, represent the main link between the major and the minor systems. Features of the dual drainage system, which are considered in DDSWMM, include "inlet control," that is the limitation of flow access into the sewers, and separate storage facilities for the minor and major drainage systems (dual storage). In DDSWMM, the major system network and the minor system network need not be parallel or flow in the same direction, which adds considerable flexibility to the model.

Inlet Control: A tacit assumption in existing urban drainage models is that catchment runoff is transferred, unrestricted, through hypothetical inlets, into the underground sewers. This is a simplistic conceptual approach which ignores the limited capture capacities of storm inlets, may result in a false design and precludes the use of these models in applications involving major rainfall events.

DDSWMM provides a realistic design of urban drainage systems, by including the hydraulic analysis of storm inlets. Runoff in excess of the capacities of storm inlets is routed through the major system network to discharge points, storage facilities, etc. DDSWMM gives the user several options to extend the drainage system design to cover rare rainfall events, including the determination of additional inlet control requirements to protect the minor system from high surcharge levels.

In addition, DDSWMM can also simulate major system flooding at street sags or depressions, a condition that may exist in older developments, not designed according to the dual drainage principle, and aids in the design of relief measures.

Dual Storage: An extension to the dual drainage principle is Dual Storage, which refers to separate storage facilities for the minor and the major systems. Runoff detention is a method widely used for limiting post-development flows, among other applications. DDSWMM has several options for the design or evaluation of such storage facilities, either as separate units for the minor and the major systems or as storage units for the combined flows from the major and the minor systems.