64 bits & 32 bits ports

Hello, Is there a mean to build ports for x86_64 architectures and i386 ones together? If not, how to specify macports to build ports with the option "-arch x86_64"? Indeed, for some ports a "universal" variant exists, but it seems to be only for ppc and i386 architectures. Thank you in advance for your help. Sincerely, François Sausset