Dave,
The most important thing is to make sure the camera is on the same settings for each pic.
If you take a pic of two different lights, on two different settings, the two pics are meaningless when compared to each other.
This means you can NOT use the cameres auto settings.
The main thing is the exposure,
I like to set mine to the brightest light, so it is just right.
You dont want your brightest light to be TOO bright, so its one huge beam, instead of being able to see the hotspot, and corona seperately.
if you cant set your camera to a fixed manual setting (which i am sure your camera can) the trick is to take one pic, with both beams in the same pic. So, that way, they are at the same setting.
Like I said, just make sure that the setting you use, is the same for all your shots, that way, when you compare them to each other, they will be correctly exposed.
-John