The WH7 provides 4 output leads capable of providing 54W each. Each lead is load sensing, so if a bulb draws 9W, it will only provide it the 9W it needs. Put two of the output leads on a single bulb and both will sense that the bulb needs 9W and it will provide 18W - 2x OD (though not exactly a 200% increase in light output).
I've read that the Icecap ballasts are able to OD bulbs "better" (There was one paper where they overdrove a T5 bulb with an Icecap and got the same light output as a metal halide. I've forgotten what their definition of "light output" was but it was interesting to see), but I'm happy with my Workhorse 7s.
I expect overdriving probably works better for slimmer bulbs.. I tried 2x and 3x overdriving with some T12 bulbs I had and didn't think the output increase was appreciable at all.. They got a whole lot hotter, but the added light was piddling.
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