What are the chances that two messages have the same MD5 digest and the same SHA1 digest?

Question

Given two different messages, A and B (maybe 20-80 characters of text, if size matters at all), what is the probability that the MD5 digest of A is the same as the MD5 digest of

Answer 1

If message size is not restricted, the chance approaches 100% asymptotically, as there's an infinite number of possible messages and a finite number of possible hashes.

(note: edit to question makes this less relevant now)