As I understand it (and I don't understand it very well), it depends what you're doing. The way I learned it was to think of it kind of like this:
If the program is just a linear set of instructions such as first do A, then do B, then do C and you have to complete A in order to do B (perhaps if the result of calculation A was used for calculation B), then having multiple processors doesn't help all that much. If we assume each step takes one cycle, then the entire thing will still take three cycles (keep in mind that I am simplifying grossly here).
However, if the program is multi-threaded and requires the computer to do A and B in no particular order before doing C, having multiple processors helps. One processor would do A. Another would do B at the same time. Then one of them would do C. This program would take three cycles with a single processor but only two cycles with a dual processor.
Now if the program had four steps that all could be completed in any order, a single processor would take four cycles while a dual processor would take two.
Therefore, you can't say that a dual processor is twice as fast as a single processor or is 1.8 times as fast or anything like that. You can give an average, I suppose, but that's not very predictive.
I hope I didn't mangle all that too much.
