I think the contraction "we'd've" for "we would have" is disallowed, but it doesn't seem technically incorrect. It seems to work well with common phrases like "we'd've gotten killed out there".
Of course, it's common in spoken English, but single contractions are quite common in written English, so I can't really see any reason that doubles shouldn't be allowed.
Is there a non-historical reason that it's invalid now, other than that it's not in any dictionary? Could it possibly become valid over the course of time?