The key to this idea is that it is old
. I occasionally see this in the main stairwell of early 1900 two story single family dwellings wired with Knob and Tube. The original service was 120 volt only and a lighting circuit went to each floor (or there was only one lighting circuit for both floors). The main stairwell light is not polarized (like I said - OLD
. The lamp screwshell is connected to the common of one threeway, and contact tab is connected to the common of the other threeway. The threeway travelers, at the top and bottom of the stairs are connected to the closest available hot and neutral. No concern is shown for cross neutralling (is that a word?) two circuits.
I generally get a call when the home owner has tried to replace a worn switch and gotten the common on the wrong wire. . .always blows the breaker.