At the moment, we know nothing about their connection that's why we speculate.
It's hard say that Obito and Madara knew each other as Kishi said.If you read what Madara sometime after he was revived via Edo tensei,you'll see that he said 'I awakened this renigaan shortly before my death' .Now we all know Tobi was preaching Madara's history as though he(Tobi) was Madara.
If Tobi is Obito and Madara awakened his eyes shortly before he died then that doesn't make sense because his eyes were then implanted to Nagato;who is a good few years older than Kakashi and Obito.
More questions arise in this case:
1-If Madara awakened his eyes shortly before he died,then who implanted them?If it was Zetsu,then why didn't he go back to Madara when he was revived ?
2-If Tobi is Obito ,then how does he have vast knowledge about things like Susano'o, MS,EMS ,Amaterasu...etc Could it be the black half of Zetsu?Because he seemed to have exceptional knowledge about these kind of things as shown when Sasuke and Itachi where fighting.
That could mean Zetsu has a bigger role in this manga than we thought. :shock:
That could change everything!