I've seen a contraction of two words. I can't see why it wouldn't've been possible to have been contracted twice. Is it possible and how should it be punctuated?
Update: Ok, to sum up the answers so far
- This appears in spoken British and American English
- It is from one of the lower registers of English
- Even if spoken this way sometimes, it isn't really written as a double contraction, except as written speech in fiction.
- And from my own googling in Wiktionary, it appears most written forms are old British words, often nautical like fo'c'sle.