The practice of the generation stage is to purify phenomena primarily through visualization of the buddhas, bodhisattvas, and their mandalas. The practice of the completion stage is to realize emptiness with practices based on the channels, wind-energies, and essences of the subtle body. To practice the union of the generation and the completion stages is to visualize the yidam (the meditational deity) as an illusion, as a dream. Normally the proper sequence of practice is to practice the generation stage first, then the completion stage, but there is no need to follow this sequence in the practice of the union of the two stages. From the standpoint of phenomena, it is the generation stage; the dream-like, illusory emptiness represents the completion stage.
~Depicted from GATEWAY TO VAJRAYANA PATH - The Generation Stage