This paper presents the dynamic characterization of two similar multi-span viaducts, supporting the two carriageways of A4 highway between Padova and Trieste. The bridges are characterized by 7 spans of different lengths, steel-concrete composite girders and slightly curvilinear planimetric shapes. The main differences between the East and West carriageways are primarily related to the length of the second span; nevertheless, the resulting dynamic behaviour is quite different. With the aim of Structural Health Monitoring (SHM), each viaduct was equipped with a continuous dynamic monitoring system consisting of 42 accelerometers, enabling real-time vibration measurements. During the first days of monitoring, many key vibration modes were identified within the 0–8 Hz frequency range. The comparison of the identified modal parameters highlights the impact of span arrangement on mode shapes, revealing significant variations despite the structural similarities. These findings contribute to a better understanding of the dynamic behaviour of multi-span continuous viaducts and provide the reference parameters for the SHM of the two structures.