The co-existence of these two characters has already been established by a brief appearance of an Alien skull aboard a Predator ship in Predator 2 and in numerous comics dating back decades which, ironically, is roughly how long this film bounced...

Suffice it to say Aliens and Predators end up on earth, tucked away in the arctic with a bunch of humans, and hunt and kill each other and the humans that get in the way. Or even stand adjacent.