Yes, I agree - except with the risk calculation.
If the possible causes for two discs "A" and "B" going bad are completely independent of each other, then the probability of both discs going bad is the product of the individual probabilities; e.g. if there is 5% chance of disc A going bad and 4% chance of disc B going bad, and the causes are completely non-dependent, then the probability of both going bad is 0.05*0.04 = 0.2%. So the risk is significantly lower than half the original risk!
If the causes of failure are not independent however, like A and B being the same batch of discs burned in the same burner and exposed to the same environment, then the risk of both discs going bad could be as high as the risk of only one of them going bad!
This is why you should burn on different batches of media from different producers and then keep the two copies in separate locations!