Aslan and Tash certainly are NOT the same!! But what makes them different is not the fact that one name has two syllables and the other name has one syllable. They differ in their NATURE AND CHARACTER. Emeth, although he was surrounded by people who spoke the phonetic name "Tash," had a heart that wanted to encounter the NATURE AND CHARACTER which really belonged to Aslan.
Imagine that you had never heard of Jesus at all. Then imagine that someone told you a highly accurate account of Jesus' Lordship and everything He did for us....EXCEPT that the person talking to you used the name "Hercules" where he should have said "Jesus." Suppose, then, that you knelt and prayed, "God, I understand now that I am a sinner in need of grace. Please forgive my sins and change me into one of Your children...in the name of Hercules, amen." Do you think that God would refuse to recognize your prayer just because you used the wrong phonetic name? No doubt He would see to it that you got that error corrected; but you would be a Christian from the moment you uttered that prayer, because you prayed in the name of the NATURE AND CHARACTER which are the true nature and character of the actual Jesus.
What Mr. Lewis did with the character of Emeth was a more liberal stretch than what I have just imagined; but surely you get the idea. Note that, as in my illustration, Aslan did not leave Emeth to go on thinking Tash was God; but He recognized that Emeth had already been responding to what John Wesley would have called "prevenient grace."