I don't think probability works in the way you imply. ACARS fails quite regularly, once ACARS has failed what is the probability that the transponder fails? The same as it always has been for transponder failures. Unless there is a common mode failure in which case it is much higher.
Say the odds are 1 in 1,000 that the transponder fails and 1 in 1,000 that ACARS fails. Then the odds that both fail at the same time are 1 in a million. Easy math.