OK, first the answer to the easy one:
To run 2 bulbs: you buy an extra light fixture, and put it in the canopy! ... <assuming you have room>
Ballast is a transformer that takes the 110V house current & step it up to several thousand volts the flourescent bulbs needs. <You'll find that inside your canopy.> Each ballast is rated for a specific watts / number of bulbs. You can't just wire extra bulbs to your existing ballast, nor can you put in a higher wattage bulb. Some people have wired a ballast rated for 4 bulbs & connecting that to 2 bulbs - in effect doubling the volts to the bulbs. This gives more (but not double) light but will burn out the bulbs in a few months. This is called overdriving, but I am not recommending non-code wiring of electrical stuff, esp. over water. Also, after buying the new ballast & taking into account the number of bulbs you will be going through, it is prob. cheaper (& definitely safer IMO
) to go with power compact.