I'll just bet that the 3-way travellers go from switch to light to light to switch, and that there are only three conductors between the two lights.

In this threeway scenario, one needs four wires between the lights for each light fixture to be in parallel with the switched leg and the neutral.

I'll bet when the original installer found his(er) mistake, the two fixtures were put in series with each other and connected between the switched leg and the neutral. Some light is better than no light (and maybe no one will notice [Linked Image] .

Al Hildenbrand