One Variac powered with DC. I used a variac (or autotransformer) to power two lightbulbs designed for 12 volts and 5 W from a 12 volts battery. This method works to regulate the intensity of each lightbulb in opposite way. But I have noted that this method presents a complex dynamic response: it seems that two effects are acting simultaneously. One effect acts very quicly to change the intensity. A second effect seem to react a bit slower that the first effect and it rebalances the final intensity to each lightbulb. You may only note this second effect when moving slowly the roll in the variac, when you stop there is a second action changing the light intensity after some tenths of second. If you move quickly the roll back and forth there is not enough time to see this second effect. In summary the dynamic response seems to be quite complex, maybe because of different magnetic fields fighting into the toroidal core. Only using about 1/4 or 1/2 of the total variac length I got a total regulation of the light intensity between the maximun light intensity and almost zero light intensity.