Using a custom form_validation function to check two fields

[eluser]Unknown[/eluser]
Hi there! I'm new to this forum, and have only two hours of experience with CodeIgniter... I'm working on it though, and working my way through the User guide (actually creating a small application).

I've come to the topic of form_validation now, and there's one thing I want to know:
How do I pass two field values (for instance 'username' and 'password') to a custom function?
Say, I would like to see if the two exist in one table row. I've read how to make a custom function, but can't find a (or the) way to use variables of two fields.

Anyone?