Chai = Life = Yod + Heth = 18 The most common spelling is "Chai", but the word is occasionally also spelled "Hai." The usual modern pronunciation of this word (in terms of IPA symbols) is [xai], while a transcription of the Biblical and Mishnaic pronunciation would have likely been [ħai] (with pharyngeal consonant). http://en.wikipedia.org/wiki/Chai_(symbol)
The number 36 is twice 18. In gematria (a form of Jewish numerology), the number 18 stands for "life", because the Hebrew letters that spell chai, meaning "living", add up to 18. Because 36 = 2×18, it represents "two lives".