Why does the Northern polar vortex usually have two centers?
Northern Hemisphere

The spatial structure of the polar vortex is determined by the stationnary waves. Stationary waves in the northern hemisphere are primarily modulated by stationary (surprise!) forcings of the northern hemisphere : 


Topography : Rockies and Himalayan mountains
Land-sea ocean heating contrast


This spatial pattern is mostly locked but they are slightly modulated by the seasonaly changing meridional temperature gradient and from internal chaotic processes (i.e. turbulence).

This article is a must-read and not too technical for the northern hemisphere circulation:

http://onlinelibrary.wiley.com/doi/10.1029/134GM01/summary

vs Southern Hemisphere

When you compare such forcings between the northern and southern hemisphere, there is a big difference: much less land-sea contrast and less topographic forcing in the southern ocean. Hence, a "wavenumber one" configuration, as compared to a "wavenumber two" for the northern hemisphere.