the most current sizing requirements i've seen are "2 sided, 12 square inches = 1 cube"

if this is 24 square inches, why is the 2 light recommended only for 1 cube? because of spread?

Also, is it possible to add lights at a later date? I imagine this is a "duh" question, but i'm thinking more along the lines of, if i choose to add more lights at a later date will i have to drill holes and such, or are there markings/brackets inside the canopy that would let me place them in the right spot without having to do a bunch of measuring.