Here is the reply we got from Fulham. Wiring diagram 11 is for parallel wiring of 2 tubes.
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Presently the Workhorse line of ballast will only provide enough lamp current to operate one VHO lamp at a time. Some people have attempted to operate two lamps wiring two red lead to each lamp with the yellow lamp in common, this will only provide enough lamp current to operate the lamps at about ½ light level. Some people use wiring diagram #11, but it defeats the purpose of using the VHO lamps as this method only provides the lamps with around 800mA and operate it like a HO lamp. If you try using all four red output wires, the application will overdrive the lamp operating current by as much as 20% which over-heats the cathodes. This is not an approved Fulham application and it voids any warranty of the product itself. We do not assume any responsibility for lamp or ballast system operations.
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We also got this comment in another respone from Fulham:
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In addition, Lamps should NEVER be wired in series. Running the bulbs in series would reduce the total number of turn-on cycles. There is no correct estimate of how much reduction because it depends on the lamp types and its wattages. If the single lamp voltage requirement is close to the ballast starting voltage, the addition of the second lamp will cause problems. We do not recommend series wiring with instant start ballast, however, if you have a continuous burn application then series wiring of lamps is ok
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At least one manufacturer using Fulham ballast (CoolTouch lighting) has changed to use one ballast per bulb.
I'm still using the series connection but will be looking into switching over to one ballast per bulb.