Are you absolutely sure that they cannot run at the same time? Simulation time and animation time is two completely different thing. i.e. even if you see that message animations of two messages are sequentially animated on the GUI, it does NOT mean that they are not happening in the same time in simulation time.
Watch the simulation time in the upper right corner and see when it is progressing and when not. Or (if you are more of a visual type) record an event log from the simulation and see the sequence chart generated from that run. You will easily see events that are happening at the same simulation time.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…