As you found out, it's a common problem with the original 5D (a recall was done, plus it was remedied late in production and on subsequent models). It's pretty surprising for it to occur on two cameras within minutes of each other!
If you are shooting professionally with your Canon, you would be advised to register your gear with Canon, perhaps join CPS. Then you'd get announcements of issues such as the 5D mirror failure well in advance and when Canon is offering a fix under a recall (ended now, but they will still repair after a failure).
Also, most pros have their gear "tuned up" every year or two, depending upon how heavily they use them. The problem might well have been caught and fixed during one of those tune-ups, rather than failing on site at an important job.
I am not trying to cut Canon any slack... It was their screw-up with the 5D classic mirror, that it wasn't more securely and properly installed to begin with. But they have to be given credit for having gone to great lengths to notify people about the problem and fix it proactively. They issued a recall notice to owners who registered their gear, and publicized it a number of ways. The problem has been openly discussed widely on the Internet, including here on POTN and every other major online blog, magazines, professional trade associations.... a lot! IMO you'd almost have to be living in an alternate universe to not have heard about it!
I have to lay a little responsibility on anyone using an 8 year old camera for work, that they need to maintain their gear in a professional manner, too. It's simply part of "doing business" as a photographer. If OP has been getting their gear serviced regularly, I'd be looking around for a better service tech! Camera repair techs have their own networking and discuss common problems and fixes a lot among themselves. The 5D mirror issue is bound to have been discussed, especially considering how popular the camera has been among professionals, especially wedding photogs.
It does suck, to have two cameras fail at a wedding! Been there, done that. If there is any place that camera gear is most likely to fail, it's at a wedding. Maybe it's all the angst in the air.