To add what Jim pointed out, basically you rewire the fixture so that a two-bulb electronic ballast drives only one bulb and add a second ballast, rewired the same way, to drive the other bulb. The ballasts actually run cooler because they have less load. The bulbs run warmer and don't last as long but overdrivers think the brighter light is worth it.
This is some what different issue with MH (with the exception of Radiums), MV or HPS. Screwing say a 100 W into 250W or 400 W ballast is a no,no. The small W bulbs plasma arch tube gets gets over heated do to the increased current, causing the internal arch tube vapors to rise to high, thus increasing the internal arch tube bulb pressure, often making the bulb explode. And yes the bulb is brighter. If you took any arch bulb lamp, be it HID or LID and ran it direct, you would get run away voltage, as the bulb would draw in so much current the bulb would destroy itself by burning up the cathodes. The reason behind a ballast.
Here is another example on HID lamps
Some may be offering a “50 watt†system. 50 watt HID Xenon bulbs DO NOT YET exist anywhere for sale 50 watt ballasts do exist but are quite rare and are not generally on the market either (we could offer 50 watt ballasts but will not until our 50 watt bulb is ready). Some other suppliers are using standard 35 watt bulbs with a 50 watt ballast (a shortsighted mistake) they overdrive the 35 watt bulb, greatly increasing the likelihood of bulb explosion (overdriving is not covered under warranty). Overdriving cuts the life in half and the light output (brightness) drops off early and significantly with use over this shortened life. This is not the "right" way to do it and not the way we are doing it (like getting a wine to market before its “readyâ€). We are doing prototype performance testing on a real 50 watt bulb/ballast system as we speak. The output of our 50 watt bulb is 75% greater than the 35 watt model (5,700+ lumens compared to 3,300 lumens for the 35 watt and our light output is sustained) The 50 watt system will be even more efficient, at 114 lumens per watt compared to 94 lumens for the 35 watt system. We should have this product ready within about 6 months also. It will be a 50 watt bulb and a 50 watt ballast both designed for 50 watt operation, with a full warranty. The power consumption will be less than 60 watts total or 4 to 5 amps at 12/14 Volts and 2 to 2.5 amps at 24/28 Volt
I might add that there are even guys that bulb fashlights with overdriven bulbs