I believe it's all in the wattage. For example, if you have a ballast that is rated for 110 watts, you can run a single bulb that is rated for 110 watts OR you could run 2 bulbs that are rated for 55 watts.
The wiring schematic would change for the different configurations of bulbs. For the 110 watt bulb, you would have a hot wire attached to one end ( both posts ) of the bulb and a ground to the other end ( again both posts)
For a dual 55 watt setup you could actually have two ways to wire it.
The first would be to "split" the hot and ground wires and wire the two bulbs just like you had two pairs of wires. You could look at this a wiring the bulbs "parallel".
I have heard ( but not tried ) that the second way works also. In this setup, you would run a hot wire to one bulb ( two posts ), a jumper wire would connect the other two post to two posts on the next bulb and the ground would be connected to the remaining two posts on the second bulb. This would be running the bulbs "in series".
You can overdrive bulbs but it will produce extra heat ( but it will also produce extra light from a non VHO bulb). You can safely overdrive a bulb up to 4 times its rated wattage but you hit a "point of diminishing returns" at twice the rated wattage. After that, you are just basically creating heat with little extra light. Many folks think that if you overdrive a bulb that it will explode. Nonsense!
If you are going to try something like that, you might want to get an IceCap ballast. They are expensive but they have a starter circuit that can help your bulbs last longer. It is not an "instant on" starter circuit so it brings your bulbs up more "gracefully".
I run overdriven "daylight , 6500 )bulbs but I buy them from the hardware store and they only cost about $3.50 each so even if they only run for half as long ( but they DO last longer ), I'm still money ahead.