This paper discusses the use of Thing Artifact (TA) and Digital Twin (DTs) concepts to engineer IoT applications. On the one hand, a TA is a chunk of unstructured information capturing 3 cross-cutting aspects referred to as functionality, life-cycle, and interaction flow. On the other hand, a DT represents a TA in the cyber world and is devised along 5 perspectives referred to as functionality, time, resource, data, and trust. To ensure a free-of-conflict environment between DTs, work-with-me, work-for-me, back-me, and avoid-me relations are set identifying who can work with whom, on what basis, under what conditions, and for what purpose. Building upon these relations, a set of guidelines to engineer IoT applications are presented and then, demonstrated through both a case study and a system.