Yes. There have been times where there were more than one imam. And presently, there is no imam. However, we do not hold to the doctrine that there always have to be one imam on earth, at least if you are referring to the 12er doctrine of al-Hujjah. Otherwise, we would be in trouble now that there is no imam manifesting his authority today. We also do not hide behind the occultation doctrine as do the imamis. Instead, we await for a just Fatimi sayyid to manifest himself and call others to obedience. Once this person does so, we will follow him, inshaAllah.
That is not to say that we would follow just anyone! He would have to meet the stipulations of the imam, as per mentioned in some of our texts.
Maybe you are referring to the well-known hadith: ((Whoever dies without knowing his imam, dies a death of Pre-Islamic Ignorance)). When one of our imams, Imam Mansur Billah al-Qasim bin Muhammad, upon him be peace, (b. 967 AH) was asked about it, he said:
The intended meaning of that is that it is obligatory for one to know the imam that one is to follow and be guided by, whether it is by notoriety or personal experience. If he [i.e. the imam] does not manifest himself, it is obligatory for one to know all of that which mandates one to be followed and wait for his appearance, be ready to obey, help, and advise him.
When one becomes morally responsible, one has to discover that by contemplating the proofs and by asking the righteous amongst the Descendants of Allah’s Messenger, peace and blessings be upon him and his progeny, as well as their partisans (shi’a), may Allah be pleased with them.
If one does not do that, it would be as though he died the death of Pre-Islamic Ignorance, as mentioned in the report.
So, according to the imam, upon him be peace, the imam not being manifest is not a precondition of knowing the imam. One has to be familiar with the preconditions of the imam as evident in our aqeeda texts. You can refer to our text, “The Precious Necklace.” http://www.scribd.com/doc/37766625/The-Precious-Necklace