If there are two masters on the same bus, there are arbitration procedures applied if both try to take control of the bus at the same time. When two chips try to start communication at the same time they may even generate a few cycles of the clock and data that ‘match’, but eventually one will output a ‘low’ when the other tries for a ‘high’. The ‘low’ wins, so the ‘loser’ device withdraws and waits until the bus is freed again. Once a master (e.g., microcontroller) has control, no other master can take control until the first master sends a stop condition and places the bus in an idle state.