We'll never know the full extent of her involvement, but we can be fairly sure she was in no way a co-president. He was the man.
Some things never change. He is still the man, and if she becomes president, the next Clinton presidency will feature markedly different dynamics from the first. The Constitution will say she's sole president, but Bill's constitution will say otherwise. Unless she is weaker than many assume and totally dependent upon him, she will surely find his irrepressibility and narcissism a powerful and distracting annoyance.
Can you envision the anarchy that could ensue on Pennsylvania Avenue? Other than "24's" fictional President Logan, did any other U.S. president have to enlist the Secret Service to sequester his spouse?
Apart from the "24," scenario I'm being quite serious about this. Bill Clinton is not constitutionally constructed to play second fiddle. Try as he might, he cannot help himself.
Even if he wouldn't vie for direct power -- which is unlikely, considering he's already lobbying for it with his ill-considered quip that Hillary would appoint him and Bush 41 as world ambassadors -- it's inconceivable he wouldn't always be trying to grab the headlines. And the media will always be ready to accommodate him.
So in the end, a Hillary presidency, in addition to the other woefully bad things about it, will be an internally chaotic, unstable mess. Unless she can muzzle Bill more on the campaign trail -- which could hurt her prospects as well -- voters are bound to consider these things if she wins the nomination and they more soberly contemplate an actual Clinton co-presidency.
She's already declining in the polls, in part due to the perception that she is unelectable in the general. The thought of a truly two-headed presidency, especially with these dual snapping turtles, will only add to that perception -- to Hillary's further detriment.