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It could be two names. Koumis is certainly a Greek surname, meaning unknown to me, unless it's a shortened version of Iakoumis, a familiar form of Jacob or James.
Kertanides (I'm assuming a missing second E)would mean "son of Kertanos, or Kertanes, or some such. I can't find a name or even a vocabulary word that comes near to that.
Then again your ancestor might have been called (Ia)koumis Kertanides, and what you have is a forename and surname compounded.
Mysterious!