Two 5D Mark IIIs will certainly work, and will produce great results with a few caveats:
1. The speeches can't last more than 30 minutes. This camera has a 30 minute continuous video recording limit.
2. If you are used to camcorders, and autofocus is part of your shooting style, you will miss it. Sometimes, you will miss it a lot.
3. Outdoors, you will need to buy a separate monitor or a loupe for framing and focus. The Canon's LCD may be washed out by the sun – and the optical viewfinder stops working when you record video.
As long as you are fully aware of these issues, and go into it with your eyes open, the $3499 5D Mark III is a great choice.
That said, if you're going with a large sensor interchangeable lens camera, and you don't need to stick with Nikon because of a pre-existing invesment in lenses, I recommend the $1049 Panasonic GH3. It has none of the Canon's limitations: it can shoot video continuously for hours, it has fast video autofocus and its viewfinder works while shooting video. I am a GH3 shooter, and it is the best valie-for-money video/still camera on the market today.
This camera can produce wedding results like these:
Again, best of luck with your decision. Hope this is helpful,