The effect of second normal stress difference is analysed by observing the co-flow of two batches of the same viscoelastic fluid, each with a different pigmentation, in straight ducts. The development of secondary motions is observed by tracking the motion of the interface between both fluids. Numerical simulations are also performed on the basis of a multi-mode differential viscoelastic fluid model and are compared with the experiments. A good agreement is found between the experimental observations and their numerical counterparts.