Correction

This is not correct either - there is no official Slackware x86-64 port; 
the only official Slackware is the 32 bit x86 build, or the Slack/390 
port.

Anything else is unofficial and not Slackware (although we do try very 
hard to be Slack like). So technically, Slackware doesn't do multilib (and 
Pat Volkerding has stated in interviews in the past that any such official 
port would likely not be multilib anyway, so you would be right on that 
front); but then again, Slackware doesn't do x86-64 either at the moment 
either.

(I should state for the record here that I am a Slamd64 contributor).