On a Mk3, you can record to SD and CF simultaneously. On an HVX... I think you can record to both P2 slots simultaneously.

So, the option is there... but does anyone actually use it in a wedding environment? Or is the amount of data just too prohibitive?

I've always been meaning to invest in more cards to take advantage of this, but then there's always something else to spend money on...

I thought that you could only record to one card in video mode? Be cool if you could but I couldn't figure it out.

Personally I'd probably double record the ceremony and first dance/toast.When it comes to events there is never such thing as having too many backups. DJ's think I'm crazy when I'm pulling 3 different output signals from their system all on different recorders.

With the 5D3 it's still photos only that get written to both cards. The XF105/305 & the C300 can be configured to write to both simultaneously but I have never used it as they are incredibly reliable with SanDisk Extreme CF cards. I suspect that it's a more useful feature in saving time copying cards where you need to give someone a copy of the rushes.