use the text box for each trigger to name what it is to help you keep track of what trigger does what. "Person 1 line 1" or something. Then it's just a matter of figuring out at what time each text bubble is to appear, and making the corresponding trigger. It might help to work it out on paper before you start entering trigger data. Write your script, then figure the exact time each line should be said. that time is set in the "trigger start time step" field.
trigger 1: at 0 seconds, person 1bubble, text =" hi there"
trigger 2: at 3 seconds, person 2 bubble, text ="hi yourself"
trigger 3: at 6 seconds, person 1 bubble, text = "nice weather, huh?"
trigger 4: at 9 seconds, person 2 bubble, text = "if you like rain"