Is it possible to have a tie in Munchkin?
Yes that is one way to achieve a tie. A SJG representative confirmed it (with a slight clarification with regard to being enthralled):

Q: I was playing the original Munchkin game as an elf who gains a level for each monster that they help kill. I was enthralled to fight by a bard. We were both level 9 and killed the three monsters. Since we both gained the three levels we were both level 10 at the same time. Who wins?
A: It's a Level gained by killing, so the Elf can, in fact, get the winning Level by helping.
Missed the Bard wrinkle, so here's the proper official answer: The Elf couldn't have been enthralled in the first place. Had he been enthralled, before the win was on the line for the Bard, he still could have won the game, but if suddenly the Bard had the chance to win the game as well, the enthralled Elf leaves the combat outright. The Bard can still get help, but has to do it based on his real personality, not some Svengali like ability. . .