This is a good question indeed. If owls with asymmetrical ear canal openings have different arrangements, it would suggest that the asymmetry has evolved more than once and arrived at different solutions.
The Cornell Handbook of Bird Biology (2nd ed.) states that the Boreal owl right ear is positioned higher than the left. Ornithology by Frank B. Gill (2nd ed.) states that the barn owl left ear is higher than the right. If both of these reports are accurate, then we have two species that differ.
The Wikipedia entry on Sound Localization in Owls (section 3) also suggests that the asymmetry of different owl species may be a product of convergent evolution:
For example, in barn owls (Tyto alba), the placement of the two ear flaps (operculi) lying directly in front of the ear canal opening is different for each ear. This asymmetry is such that the center of the left ear flap is slightly above a horizontal line passing through the eyes and directed downward, while the center of the right ear flap is slightly below the line and directed upward. In two other species of owls with asymmetrical ears, the saw-whet owl and the long-eared owl, the asymmetry is achieved by different means: in saw whets, the skull is asymmetrical; in the long-eared owl, the skin structures lying near the ear form asymmetrical entrances to the ear canals, which is achieved by a horizontal membrane. Thus, ear asymmetry seems to have evolved on at least three different occasions among owls.
(Emphasis added)
I presume the source of the Wikipedia information to be from Konishi, Masakazu and Susan F. Volman, 1994, though it is not cited in the relevant paragraph.
Perhaps some other ornithological journals will have more specific information, perhaps even allowing us to trace these separate evolved structures. It may even become a great thesis project!
