So we've established that lux are the number of photons striking a unit area per unit time, weighted by a luminosity function.
We've also established that photons from the same light source are indistinguishable, as long as they have the same energy/wavelength/color.
What this means is that if you put two lights the same distance from a point, and each light provides N photons per unit area at the point, with two lights you will have 2N photons per unit area at the point. Because intensity is a measure of the number of photons per unit area, the light is twice as intense, whatever unit you choose to use. Twice the lumens, twice the lux, twice the footcandles.
An obvious practical caveat to this point comes when using multiple low-intensity light sources. Notice how I stated that the lights were at the same distance? A practical problem with CFLs, for example, is that while you can get 27000 lumens from 10 x 42W CFLs, it's difficult to get them close enough to make them useful. If you have them in a line, for example, as I've shown below, each successive light is further from the meter, and the effective increase will be reduced. They still add, but according to the 1/d^2 rule, so having a bulb 2cm farther away will yield diminishing returns. On the other hand, this can be an effective way of distributing light, whereas with HID you need to distribute the plants around your single point light source.
