Nice Topic...
I would say they would not so much interact, i doubt they would attack each other and i doubt they would do nothing but perform there basic instincts.... so it would be like asking how would a Maggot and Fly or a Caterpillar and Butterfly interact.
As far as the passing of DNA.....
The Face Hugger seems to pass on a Embryo that then has a basic form and set of Genetic Coding that it modifies by taking some DNA and Traits from its host in order for it to be able to habitat the environment that the host Organism survives in.
The Trillobite does the same only the Trillobite is a Human/Xeno DNA Face Huger and so the base form Embryo would have a lot of Human/Mammal Characteristics
