The short answer is no. Overdriving a lamp will not shift the output towards more dangerous radiation. To get UV B or UV C, you would need to have a specialty lamp. The normal "soft" glass (soft as opposed to quartz) that the lamp is made of will filter out almost all of the dangerous radiation.
The UV A region (315 - 400 nm) is used for sun-tanning lamps, while UV B (260 - 315 nm) and UV C ( < 280 nm) are the regions that contain the wavelengths that are fairly effective at disrupting organic molecules (like DNA) and killing cells. (the wavelengths most effective in killing cells lie below 300nm, while those most effective in producing ozone lie below 200nm.)
The region of the spectrum that is visible to humans (sunlight) extends from about 390 nm (ultraviolet) to 780 nm (near-infrared)
Most Actinics and "black lights" produce UV in the Near UV (or UV A) spectrum. This is not "that bad" as far as ionizing radiation goes. It does penetrate skin more deeply than UV B, and triggers a short-lasting tan that doesn't protect from sun-burn. It is not any more dangerous than sunlight (of which it makes up about 4% of the radiation that reaches the surface of earth).
About 1% of sunlight on a clear day in the lower latitudes is in the UV B region. This higher energy radiation doesn't penetrate the skin as deeply, but it triggers a longer lasting tan ( as well as premature aging and cancer). UV C radiation from the sun from is entirely absorbed by the ozone layer (thankfully). UV C is extremely damaging to the skin and eyes.
In a florescent lamp, light is emitted as a result of current flow through the mercury vapor between the electrodes of the lamp. The low pressure mercury vapor in a florescent tube produces the majority of its output output at 253.65nm (the 63P1 - 61S0 transition in energy level of Mercury). This is in the UV C range, and wouldn't otherwise be useful for much (except maybe producing ozone or sterilizing things). But the phosphors coating the tube in a florescent lamp convert this to more desirable wavelengths via "florescence". Florescence can't produce any higher energy (shorter) wavelengths only longer wavelengths, but it can produce a variety of colors depending on the molecules in the phosphors as the UV is absorbed and then re-emitted.
If you actually want UV radiation to leave the lamp, you can't use ordinary glass, you must use quartz . A quartz tube transmits 93% of the lamp's UV energy whereas soft glass emits very little.
Your typical "Black Light" emits "Near UV" or UV A which ranges from 350 to 400 nanometers. The lamps that actually appear a dark violet use a special filtering glass which reduces the passage of energy in the visible light range of 400 to 460 nanometers. Because of this filtering out of the blue, red/purple visible light rays, the lamp does not have the light blue color that you see in some some bug attractor lights (also emitting UV A), but instead appears a a violet/black color.
If your tube is a regular black light or actinic light, you really don't need to worry about the dangerous radiation. Unless you paid lots of money for a quartz tube or some other kind of specialty tube, the soft glass that it is made of will filter out *most* (at least 99%+) of the higher energy (UV B and UV C). Overdriving will produce more overall light, and thus a little more of everything in it's spectrum (including UV -- mostly UV A), but it's not going to damage the organisms in your tank in a way that they wouldn't experience in normal sunlight, and for which they probably have some adaptation (like phosphorescent pigments) or at least some amount of tolerance.
Good luck,
--dan
ps. I've heard of people taking the glass off of their metal halides, and just running with the quartz between them, and the energetic UV B and UV C radiation , I can't say that I would want to expose my skin to that for a very long time. I don't know, but it may not be that in-tolerable for the aquatic organisms. Ocean invertebrates were among the first life on the planet, and they've probably seen some tougher times than we have. UV C is commonly used for disinfecting water (not helpful if you were hoping for live plankton in your tank.)
when one turns on it might send a tiny singnal throught the ground which would operate the local controle of the X-10 mod causing the other one to turn on and vise versa.. easy to fix if it does happen though but will realy screw you up till you figure out what is going on 
