If you're developing Facebook apps or using Facebook authentication, you pretty much have to have two or more Facebook accounts. The problem is that having more than one account is against Facebook's terms of service, and they don't provide a developer sandbox.
You can create test accounts through the API. I actually have a terminal script where I just write "fox" and it opens up a new Firefox profile with a new Facebook test user in it.
The URL is on my dev server, it creates a Facebook test user and then redirects to it. First you get an access token from graph.facebook.com/oauth/access_token, then you use https://graph.facebook.com/APP_ID/accounts/test-users to create an account using the token you just got.
