My suggestion of 2nd pair of speakers was not for movies but just to fill the cavernous space at a reasonable level ... for background music and such. I get the feeling that is not the application being discussed here ...
This was definitely a consideration, but time is of value too. Taking the time to figure out how to extend the fronts & center to the other room seems too much. Plus there's still the expense of buying more speakers. Easier to just crank out more power from the existing.