Okay, so I have Dawnguard, which lets me have Serana as a follower. I've had her for quite some time and I decided to level some skills for free with Farkas (you know the train and trade process). I pressed the wrong dialog option on accident, which was the "Follow me. I need your help." And Farkas followed me even if I had Serana with me. I fast traveled to other places and did some tests to see if he'd stop following at one point. He didn't so I pretty much have two followers with me now. Is this like a glitch or something that had to do with Dawnguard DLC? Not asking for a solution here, just asking why it happened. Was it supposed to happen or...?

It is a glitch on the console. A interesting glitch that is! =D

no its on purpose serana is a special follower that sometimes ( during some dawnguard quests ) doesnt uses default follower system and doesnt take follower spot so you are free to take any other follower with you