What is the status of Hussain bin Ali, When Ali bin Abu Talib was Imam?

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.