Kamillions(1990)

Nathaniel Pickman Wingate has opened a gateway to another dimension using equations and equipment in his basement laboratory. His wife, Nancy, wants him to get ready for his own birthday party. He wants his son Sam to help him. Sam is up in his room looking at pictures of Jasmine on his computer, and a poster of her arrives which he puts in his closet. Although it is Nathan's birthday, the family is enthralled by a visit from Cousin Desmon, who is now a count in Liechtenstein. While Sam is away getting equipment for his father with his friend Alex, his father gets sucked into the other dimension, and a creature from the parallel universe escapes, pursued by another. The first temporarily traps the second with its spit, attacks Desmon, and becomes a duplicate, absorning his thoughts from the unconscious body. The other manages to get free, and unable to find a human to mimic, finds the poster of Jasmine...