We have carried out laboratory experiments for the phase split of a gas-liquid flow from a single flowline to a dual riser. The facility used for the experiments is the air-water loop at the Shell Technology Centre in Amsterdam. The 2” diameter loop consists of a 100 m long flowline followed by a dual 15 m high vertical section. The two risers are connected to the same separator at a platform that is operated at atmospheric pressure. The set-up includes a non-symmetric side-branch T-splitter, of which both the side arm and the run arm result in a vertical riser (Riser 1 and Riser 2, respectively). All experiments were conducted with slug flow conditions in the incoming flowline. It has been observed in the experiments that the liquid phase preferentially flows into Riser 2, while the gas phase is predominantly diverted through Riser 1. However, due to the vertical orientation of the risers, this maldistribution can result in gravity dominated flow in Riser 2. As a result, quasi-stable modes form in the dual riser system. These can imply extreme situations, like a fully liquid-filled Riser 2 or partial separation of the phases over the two risers. Each mode has a typical riser base pressure. It was observed that manual choking can influence the phase split (albeit to a limited extent), and is capable of switching the system between the quasi-stable modes. For specific combinations of valve openings and flow rates, Riser 2 experiences a severe slugging cycle. As a result, the phase split shows strong transient behaviour, either with constant cycle times or with an irregular time dependence. Also here manual choking can influence the characteristics of the cycle and is capable of completely suppressing it.