x64 package?

Hi, 

I wrote a Windows Store app in c#, and add MuPDFWinRT as reference. I could generate x86 and ARM packages, but failed to create x64 package. 

I know x86 package can run well on x64 machine. However, I need to generate x64 package so that I can do something different between x64 and x86 machines. 

Is it possible to generate x64 package? 

Thanks. 
