People suck,Life sucks - @blahblahusername

Say your first best friend (who treats you like crap when the other is around) is names Sara and the other is called Kayla.

Sara says something bitchy to you, hence treating you like crap.
"Hey, uhh, Sara?" You
If she says "What" or "yeah" then stand up, and go right into her face, "Stop treating me like crap when Kayla's around, kay? Or you won't have me to throw your sh*t on at all."
if she says "Not now" or something like that, say, "Yes, NOW. You treat me like crap when Kayla's around, and yet you expect me to be nice to you? I'm not gonna be respectful to you anymore when you're this DISrespectful to me. So stop treating me like crap and pretend you care about what I have to say, or you won't HAVE me at all. It's exhausting, and stupid. Being around you has become a freaking chore. BROOMS treat me better than you do."

and walk away.

Or you could mix it up in a way. :P

but there's no nice way to do it. everyone thinks sending a letter/email or getting someone to drop hints or tell, it won't work. You have to be straight forward, and this calls for it, bitchy. If she treats you like crap, you can't always treat her like gold. it's time to let loose and treat her to a little blahblahusername crap right back.

have fun!