Whenever talking about classic hard rock bands, it's anyone's guess as to who the leader is. While we all tend to see a band as a democracy amongst 4 or 5 different members, typically things tend to be the brainchild of just one person. On the other hand, it's sometimes better when you have a duo leading the charge.
It's always a bit of a different scenario when you have two creative minds working in conjunction with each other. Regardless of which instrument they play, these guys have overseen all the inner workings of their project throughout their vast history. With all that time spent together, you have to have a lot of compromise and a lot of patience to see it through. A lot of times, these plans haven't worked out, with many of these once-inseperable pairs becoming musical adversaries once free from their collaborative efforts.
However, what they did end up giving the hard rock world shows that they always work better together rather than apart. Many of these bands could be considered fantastic as a collective entity, but without these two musical thinkers leading the way, they would not have been half as successful as they are.