I've had the spraybar fall off one filter and the other's intake clogged at the same time before. Killed almost the entire tank of full grown SA cichlids. I'd still have them if I'd had 3.
The odds of a mechanical failure of two on the same day, let's see I've had 3 CF1200s for about 5 years I think without any failures(Vs the fluvals who I've had about 4 in less than that time period). So that's 5475 operational days without a failure given that we'll say each day one filter has a 1 in 5475 chance of failure.
That means the odds of two failing on the same day are 1 in 29,975,625. That's once in approximately 8 times as long as human civilization has existed. Not corrected for the decrease in reliability of 80,000 year old filters.