Obito feared Itachi because he knew he wasn't strong enough to kill him, or they'd both die if he ever tried.
Itachi feared tobi because he thought he was the fame madara uchiha. Even if he seemed in a weaker state, itachi didn't want to risk his life, and not get to fulfil his goal with sasuke, by attacking a legend. And, Let's be honest... Obito really sold the idea that he was madara.
In the end they both stalemated each other and was cautious about each other.
Itachi feared tobi because he thought he was the fame madara uchiha. Even if he seemed in a weaker state, itachi didn't want to risk his life, and not get to fulfil his goal with sasuke, by attacking a legend. And, Let's be honest... Obito really sold the idea that he was madara.
In the end they both stalemated each other and was cautious about each other.