Ok, after alot of reading I found some intresting facts about the Eperor title and Empires in CK2 time frame. It turns out that it was much more complicated than 2 Christian empires.
Bulgaria is most intresting example.
"In 913 Simeon I of Bulgaria was crowned Emperor (Tsar) by the Patriarch of Constantinople and imperial regent Nicholas Mystikos outside of the Byzantine capital. In its final simplified form, the title read "Emperor and Autocrat of all Bulgarians and Romans".Byzantine recognition of Simeon's imperial title was revoked by the succeeding Byzantine government. The decade 914–924 was spent in destructive warfare between Byzantium and Bulgaria over this and other matters of conflict. The Bulgarian monarch, who had further irritated his Byzantine counterpart by claiming the title "Emperor of the Romans" (basileus tōn Rōmaiōn), was eventually recognized, as "Emperor of the Bulgarians" (basileus tōn Boulgarōn) by the Byzantine Emperor Romanos I Lakapenos in 924. Byzantine recognition of the imperial dignity of the Bulgarian monarch and the patriarchal dignity of the Bulgarian patriarch was again confirmed at the conclusion of permanent peace and a Bulgarian-Byzantine dynastic marriage in 927. In the meantime, the Bulgarian imperial title may have been also confirmed by the Pope... ...14th century Bulgarian literary compositions clearly denote the Bulgarian capital (Tarnovo) as a successor of Rome and Constantinople, in effect, the "Third Rome".
To me this seems that Bulgaria should definetly be Empire tier, and that the best way to obtain Empire title in game would be to find a way to force Byzantine Emperor/Holy Roman Emperor or Pope to reckognize you as such. Either by force or by diplomacy. I dont think that to be crowned Emperor should require elimination of other Empires. It should just be REALY hard to get recognized.
There are other examples like Latin Empire, Trebizond, Serbia...
Here is the description and meaning of the title and list of Empires
http://en.wikipedia.org/wiki/Emperor