With multbooting on more than one computer I have several 3PC licences covering these and all in one MyNortonAccount.
And BONUS -- with Norton 2012 there's a new feature called Norton Management -- I think it's an optional add-on after you install Norton 2012 but I'm not sure since I've only dealt with the Beta -- and this each time you add a computer allows you to add the name of that computer (it picked up my Windows one by default but you can edit that if you want to) and thus know which of your two licences applies to which PC.
You can do other useful stuff like when you stop using a PC and want to use the activation on a new one -- no need to go to Customer Support ..... I hope they don't get lonely!
So buy those 3PC versions now especially if they are cheap -- and I see one store with NIS 2011 bundled with Norton Utilites for $9.99 after rebates etc during this week and you need to get it before the update their shelves -- since the 2011 KEYs will acivate the 2012 just as well .......