Bubble formation at two orifices with a common plenum was studied experimentally. Different modes of bubble formation were identified by means of analysis of gas pressure fluctuations measured in the plenum. Two synchronous regimes were found at low and high gas flow rates, separated by a wide range of asynchronous regimes. The effect of important operational parameters (barrier in the plenum and in the liquid, orifice spacing, water height, and column diameter) on the stability threshold of the lower synchronous regime was investigated.