I agree, I have a feeling that DH will let you run the application, and it will use both CPUs, however, if an application is designed to use multiple CPUs, it’s quite possible that it is very CPU-intensive.
If you know that your application is going to cost a lot of CPU-time, I would test it on a comparable non-Dreamhost machine first, or test it on DH servers at night, when load times are lower, and keep a very close eye on the output of the “top” command.
I can pretty much guarantee that Dreamhost will prohibit you from continuing to run the process if it starts to slow down the system. I think it’s best not to attract their attention at all unless it’s absolutely necessary. If you don’t have them watching over your shoulder, you probably have a little more leeway in what you can afford to run.