I see no reason why Win 8 should be a problem.  The only foreseeable issue would UAC interference if application and content are placed into the Programs folder. 

BTW, you can use 2 instances of the same program at the same time as long as they do not access the same network.  Poser will detect the network activity; disconnect the computer from the internet and it will work fine.  Otherwise, it will restrict multiple running copies.