Multiple Fields with a single callback using the form_validation library?

[eluser]octavianmh[/eluser]
All-

I've been investigating this all day, looking for just a wee bit of sample code to set me on my way, but no luck so far.

My question: must I use separate validators for each field? Maybe there's something fundamental I'm not understanding about callbacks here, but since I need to check the database for both the username and password, I was hoping to check both from the same callback, and have a successful login processed by one, concise chunk of code.

So.. can I pass more than one field into a callback function? I assume I can via an array of some sort, but a nudge in the right direction, or "give up" would be helpful!

Thanks!