Your first guess was right! Our advice for using 2 lights over a longer table is to pick lights that are a third of the table's width and then center each one over its half of the table.

Thank you again! Just to clarify: width is the long dimension? So a 78" long table could handle the two 20" lanterns pictured above handily?

Correct! In this case, 1/3 of 78 is 26, so lights that are 20" in width each will be fine. In the case of lights that are visually 'heavier' (these lanterns qualify, since they have such a bold structure--they look great!), it's a good idea to maybe not go right up to the limit since they tend to take up more visual space.