How do phototransistor and photodiode work in multisim ? How can one cause the photodiode to trigger the phototransistor?
You cannot use a photodiode to trigger a phototransistor.

A photodiode is another kind of light detector, with a similar function to a phototransistor. It cannot be used to generate light (without damaging it). A diode that emits light is rather unimaginatively called a light-emitting diode or LED.