You shouldnt have a second account anyway. It is actually not allowed for you to have more than one main characters. That is not only because of Strategus but also because of other things, i.e. the marketplace. So if you wish to have a second account for yourself, as one person, it is not allowed.
It is a different case afaik, if you are talking about a second person. Not sure how that would work though with the same IP. But no one plays warband in my household anyway... so I have no idea.
The thing about your characters is, that if you buy a second serial, but not changing it in the regedit, you will still have the old serial on your pc which will create new characters on that key.
Here for example:
http://forum.c-rpg.net/index.php/topic,18515.0.html . Shows that people having multiple accounts for themselves, are getting banned.
