i know why all the sudden your two-handed backhand is good. Assuming you're a rightie, you are now right hand dominant. While it's best for a righty's 2-h bh to be left hand dominant, either is better than neither. So what I'm saying is, you're right arm is trained to do all the work itself, and now it has an assistant in the form of your left hand. Practice left-handed forehands if you really want to take that shot to the next level.
Now to answer your question. :) (though i kinda did already) The two-hander needs to be extended just like you would hit your forehand. A small bend is optimal. Your left arm should be swinging at the ball as though it were a lefty forehand - the right arm is simply there for support and physically closer to the body because it is a below the left hand on the racquet handle. Check out Serena's bh. To me she is very easy to imitate. She doesn't have the most natural-looking swing, but has the core steps down.
http://www.dailymotion.com/video/x6e...rehand-s_sport