In brief, as Bell has stressed using a three act structure, the two doorways are an extension of this. A doorway should be thought of as a moment where the lead is presented two choices, stay where they are or step through the door and change their life in a way that means there is no going back.
Early in Act I it is best to have a disturbance which jolts the lead, this disturbance can then pull or direct the Lead until they are faced with the doorway. The disturbance does not need to be a huge thing, it can be quite small. A note from a loved one, a missed phone call, a car breaking down, or witnessing an accident. Any of these things can propel the Lead toward the doorway.
The first doorway must present a situation that pushes the Lead forward and will not let them return. A kidnapped loved one, a cop with an assignment. The question should be, can the Lead walk away and the answer should be, no. Through the doorway is the unknown, discomfort, danger.
One thing Bell emphasizes is that while this is a common structure, it is not something that you have to do. However, a writer should be aware that as you depart from this style, you depart from the readers expectations and might begin to lose them. Of course that all depends on your reader.