They can be as close as you want, but you lose a lot of the light from the tubes the closer you put them to eachother. A standard fluorescent tube emits light in all directions - down into the tank, sideways, straight up, etc. So reflectors are very important. Without any sort of reflector above a bulb, only about 1/3 of the rays emitted from the tube will go into the tank. Reflectors gather all that light going in the other directions and redirect it towards the tank. If you put your bulbs within 1 inch of eachother, there's no room for the reflected light to get back down to the tank. It just ends up hitting another bulb. Ideally you'd have enough room between each bulb to surround them with effective reflectors (probably about 2 tube diameters apart, very roughly speaking). Of course all our setups are a compromise between what would be physically ideal, and what is physically possible.
One thing that can help with putting the VHO bulbs closer together is that URI makes bulbs that have internal reflectors on one half of the tube, with the idea that radiation heading away from the tank will be reflected back through the tube and towards the tank. (Whether that actually works as well as a standard size external reflector, I can't tell you, but it must be better than nothing, right?)
Anyways, hope all this speculation and rambling helps you out some.
Nate
