In recent years, electrical machines are employed in an ever-increasing number of safety-critical applications, which require high power density, along with a demanding level of reliability and/or fault-tolerance capability. Multi-phase machines are generally considered a suitable option for satisfying these constraints. Among them, the dual three-phase configuration claims the additional benefit of being operated through conventional power electronics converters and control strategies. Despite the undeniable advantages and their wide diffusion, the thermal modelling of dual three-phase machines still represents an open research topic. Thus, this paper proposes the thermal analysis of a dual three-phase permanent magnet synchronous machine, highlighting the thermal coupling between the two winding sets. An experimentally fine-tuned lumped parameter thermal network is initially presented. Then, due to its relatively high number of nodes, a reduced order thermal network using only 3 nodes is developed. Finally, both thermal networks are examined and compared under several load conditions, in terms of accuracy and computational burden.