WARNING: Everything I say below may contain spoilers. Read at your own risk!

Hi! What was said above regarding the identity of Madara [Five Kage Conference] and Madara [Founder of Konoha] is correct. The reason why you can put both ninjas in the same lineup is because Madara [Five Kage Conference] is actually Obito in disguise (similar to Masked Man), while Madara [Founder of Konoha] is the true Madara.

~Jib

Jib Edit: Thank you shaitara for pointing out the spoiler =p

My bad, I added a spoiler alert xD