Firstly, if we had 2 Heads, Rock-like & Human-like, we may keep on accidentally banging our Human head against our Rock-like head!
Secondly, if we had 2 Heads, Rock-like & Human-like, our Rock-like head may come in very handy while dealing with thieves!
Thirdly, if we had 2 Heads, Rock-like & Human-like, using our Rock-like head, we may suddenly become very good at football!
Lastly, if we had 2 Heads, Rock-like & Human-like, we may now have to compete against our own head while eating food!