It's not a policy. CBP is to deliver all P2 with P1 as long as doing so won't jeopardize other P1 packages. If you have to make 2 trips to prevent/reduce P1 service failures, then make 2 trips.
That said, there's a trainer in Collierville who tells couriers to split up P1 and P2 packages all the time with absolutely no exceptions. And you damn well better take anything and everything you hear in that classroom as gospel, no matter how ridiculous it is. The trainer can't be fired, is treading water until retirement, and makes no effort to hide either fact.