Surprising little info on this out there. I’ll tell you the things I can fairly confidently say.
The main divider between the 2 subspecies is the Rocky Mountains. Screech Owls don’t migrate, and when they leave the nest as juveniles, they still only go a mile or 2 (<3 km) from where they were born. Having an important physical barrier, probably limits the majority of commingling.
Red morphs and the brown and grey ESO will mate. They don’t seem to worry about the difference in coloring.
Probably the main way they would tell each other apart, since they stay hidden as much as possible, is by their sounds they make when they do want to be found. They both have very distinct calls.
The Eastern has a trill sound, and also a horse whinny sound.
So while I’m mostly sure they could tell each other apart, if they happened to be in the same place somehow and took up an interest in each other, I couldn’t say exactly what would happen. I’m not sure if the visual cues would cancel the auditory ones or not for mating purposes.