you cant run both a 4 ft and a 2ft light off one ballast. usually when there overdriven there using one ballast per light, or all tubes that are the same wattage.
theres no such thing as a 20" bulb, but your right, a 24" VHO is 75 watts.
i Prefer VHO over CF as i think the color looks nicer and its not as easy to break the bulbs.