Upon seeing Dave the Silverliner's 2x F14T12 incandescent ballasted floor lamp, it surprised me that two F14T12 lamps, which add up to an arc voltage of some 84V, were able to reliably start and run on 120V, when a single F30T12, which is around 77V, generally had a difficult time doing the same. I tried a similar setup with an F14T12 and an F15T12 (total arc voltage 87V or so), and indeed, they fired up no problem. So, I tried an F15T12 and an F20T10, and, without too much trouble, they fired up. Arc voltage was 102V! You can see the incandescent ballast barely glowing as a result of seeing only 18V. I guess this works because the arc doesn't have to jump across a single set of cathodes?

How this is wired up isn't too much different from a single-lamp preheat circuit. One pin of one end of one lamp is connected to power, after the ballast of course. Then one pin of the other end of that lamp is connected to one pin of one end of the second lamp. Then one pin of the other end of the second lamp is connected back to power. Then, two starters are connected, one between the remaining two pins of each lamp. However, to my knowledge automatic starters don't work well in a series preheat circuit like this - I believe fixtures with a setup like this had a single manual preheat switch which did both lamps simultaneously. For this setup, I used an automatic starter on one lamp, and manually connected the second lamp for preheating. Then when the first lamp struck, I disconnected preheating from the second lamp.

Note that a higher wattage incandescent bulb had to be used for this setup. While 60 watt bulb works for a single 14, 15, or 20 watt lamp, a 75 watt bulb is required for two lamps.

I next plan on trying this across three, maybe even four, F4T5 lamps.