If Hussain is Imam, and Ali is also imam what is the status of Hussain when Ali is imam?
Searching Google isnt helping in anyway.
I'm asking this because , one of the members commented that muhsin bin ali died as infant when prophet was alive.
Edit: I'm editing this after the first answer, was hussain at alis imamate 1. Ordinary shia only. 2. Son of Imam only. 3. Impending imam. 4. Ordinary sunni. 5. Companion of prophet only.
Muhsin ibn Ali (as) was not contemporary to the Holy Prophet (sawa). Sunni sources apparently don't give a date of his death. According to the Shia he died before birth due to a miscarriage that Lady Fatima (as) sustained during attack on their house. But Sunni sources are mostly silent on the cause of his death. See this Wikishia entry for greater information.
And this hadith by Imam Sadiq (as) answers your main question:
حدثنا محمد بن الحسين عن الحسن بن محبوب عن العلاء عن عبد الله بن ابى يعفور عن ابى عبد الله عليه السلام قال كان على بن ابى طالب عليه السلام عالم هذه الامة والعلم يتوارث وليس يمضى منا احد حتى يرى من ولده من يعلم علمه ولا تبقى الارض يوما بغير امام منا تفزع إليه الامة قلت يكون امامان قال لا الا واحدهما صامت لا يتكلم حتى يمضى الاول. (Basa'iru d-Darajat of as-Safar al-Qumi)
Abdu l-Lah ibn Abi Ya'fur said that Abu Abdi l-Lah (Jafaru s-Sadiq) said: "Ali ibn Abi Talib was the knowledgeable Imam of this Ummah and the knowledge is inherited. Not one of us passes away until he sees a son who learns his knowledge, and the Earth doesn't last for a day without an Imam from us, with the Ummah seeking refuge towards him." I said: "Can there be two Imams?" He said: "No, unless one of them is silent and refuses to speak until the first one passes away."
The hadith implies that if there are two individuals that can qualify as Imam, one of them doesn't speak out. This is to say that only one would be Imam and the other would follow his lead until the first passes away.
This was indeed the case with Imam Ali, Hassan and Hussain (as). When the previous Imam was alive, the other didn't claim imamate.
The hadith also states that the new Imam will inherit knowledge of the previous Imam. From the Shia traditions we understand that not just the learned verbal and literal knowledge of the Imams are inherited to the next Imam, but also their attachment to the supernatural source of their knowledge (the Holy Spirit, Lawhun Mahfuz, the Column of Light, etc). This is actually how the knowledge of the Prophet (as) was transferred to Imam Ali (as) in the first place. The transfer of the supernatural source of knowledge to the successor or its consolidation usually takes place at the end of the life the passing Imam, be it a prophet or a successor. So the transfer of this supernatural attachment marks the termination of the ministry of one Imam and the start of the other, which is the reason why there can be only one active Imam at a time.
