Again I am discussing XP so Vista may be slightly different.
There is no problem with having two users as "Administrator".
The problem with setting one user as a "limited user" is that it does limit what they can do (as you would expect).
A limited user cannot install software for example, or access certain Windows system files, or install a printer (and other things).
So if you set your partner as a "limited user", and you are not around, if they need to do some system related jobs then they cannot do it till you return.
I know that Vista changed things in this area.
In XP the main user became the Administrator. Microsoft did not like the idea of everyone being a high powered administrator as a default as it allowed viruses and other dodgy programs to install themselves.
So in Vista they created a "hidden" high powered administrator acount, and a new "normal administrator" account, which is what you probably are.
More here:
http://www.computerworld.com/action/article.do ?command=viewArticleBasic&articleId=9015738&pa geNumber=2