How likely is it for timing chain and transmission to fail at the same time? And for this to occur without a warning light coming on?

Not likely but anything is possible. How about some details?

Jim, My wife drives the car... she claims the car would make a clanking noise when in drive (but at a stop). In park, you would hear nothing, in neutral, nothing... and she says she didn’t hear the noise in drive. I started the car in the garage and agree with everything she observed. I did not take it for a drive so I can’t say (myself) whether or not there was a noise while driving. Again no warning lights appeared. Had it towed to the dealership & they claimed it was a timing chain issue. We had them fix that, and now the dealership says there is also a transmission issue that was masked by the timing chain noise. They now want to do 6 hrs of work to determine the issue with the transmission. I have a feeling they are going to tell me the transmission is shot. The car has ~68,000 miles and is just days over 3yrs old. My wife is in the process of reaching out to GM. Unfortunately I have had the timing chain fail in the past, but on those occasions it took the engine with it. But those incidents occurred on older cars with twice as many miles...

Jim, Thanks for your insight. Anything relating to mechanical issues is certainly beyond my understanding. But I was suspicious of a misdiagnosis and several mechanically knowledgable people seem to doubt the original diagnosis just as you have suggested. Again thanks for your thoughts!