A lot of people have already said it, but we can't underline it enough :

JUST FORGET THIS IDEA !

It's just common sense :

• It's illegal (even if you don't use it), and some people where treated as criminals for far less harmful behaviour...
• Partnership is mostly a matter of trust, how do you want your partner to trust you ,if one day he discovers what you want to do.
• As you already pointed out, it's not only dangerous but useless (beccause of copy). and IMHO (dangerous AND useless)=stupid

That being said, I'm among those who believes that a lot of things get better
when you're prepared for the worse.
And if a lawyer is definitly the right person to talk to, in your situation, I'd just suggest :

• Copyright everything possible (picture,code,layout...)
• Discuss with your partner to put a legal framework on your partnership.
• Try to get more involved in the business (and to be fair, let him get involved in the techical things...) It's anyway a good thing to get more and more business contacts...
• If you really want to be paranoid, deploy some crucial parts of your backoffice on another box (put the database elsewhere, or put needed scripts on another box) where nobody else has access...

"Only Bad Coders Code Badly In Perl" (OBC2BIP)