It is virtually impossible to overwork a dual proc machine. thats the main difference with a mono one: Even if you have a very heavy task, the machine stays responsive.
Even if Metro is single threaded (wich is is no longer now on OSX) the work done by the system on the audio chain is spreaded on two processors. For example, the driver can run on one, and the application on another, at the same time.
Note that the choice of where the program runs is not done once and for all, it is done several hundreds time a second, so any tasks always run on the less busy processor.
I've been using dual procs macs for over 3 years now, and I wouldn't trade them for any amount of snazy leg-warmers, even with 17" screens