Mac OS X is able to use 2 or more CPUs simultaneously, just as any other UNIX/Linux system is. If an application itself doesn't use both, it's because it hasn't be programmed to do so. An application has to be programmed to use both CPUs at the same time, which most of the big programs in the professional world do since a long time already. Apple is producing dual CPU computer since a while, and applications like for example Photoshop or Logic Audio take full advantage of that ever since.
If the OS would only use one CPU at a time, there wouldn't be any speed difference between a single and a dual CPU system.