I'm not familiar with all the owls in your area, but it sure sounds like a great horned owl. Then don't all have the same pitch. If the second one has a very high pitch, it could be a juvenile great horned owl, calling to it's ma or pa. It's a great sound, isn't it?

Actually, sounds like a mourning dove to me. They sound almost exactly like owls but are a lot more common.

Great horned owls have distinctive voices. They differ in pitch and some are mellow and others raspy. Each one seems to have its own rhythm, with some stuttering sharply, others more flowing.

The calling back and forth is very common with pairs claiming territory. A few nights ago I could hear 6 owls scattered across town calling back and forth.

Mourning doves don't call at night, but owls sure do.