Check your by-laws to see if there's a way to remove your co-president. But make absolutely sure you have all your facts straight. This can be a very difficult and divisive thing to do.
I had a co-president who was very destructive. I did address my concerns directly with her (with witnesses) and listened to her complaints as well. The result was that we were dealing with an irrational and unstable person who changed the story at every turn. Basically, we tried to make a plan together for how to improve the situation. OF course, she couldn't stick to the plan, and resigned of her own free will. It was worth waiting it out though, even though it was an extremely hard time for everyone involved.
Good luck!

Has anyone ever had to oust a co-president? We have a person who is wrecking our Board with personal attacks when she does not get her way, lying and basically delegating all jobs to others and not pulling her weight. When confronted with the situation - she just shrugs it off. Our Principal even spoke with her and she still would not step down. When told that no one on the Board wants to work with her, she just ignored it. Any tips from anyone who has experience "firing" a volunteer?