Whilst the 7D is a good body, I would avoid using it alongside a 5D as it isn't backup at that point - it is just another body.
For example the 24-105 is a great lens for the wedding - wide enough for group pictures and long enough for portraits. However on a 7D it just wont do the groups
In low light and low light focussing (reception) the 7D just is not up to it and has less scope for high iso
So I wouldn't mix ff and crop.
Two bodies of the same model are preferable as the control layouts are different which can lead to missed shots - not good at the wedding 'excuse me can you kiss the bride again' is not the cry you want to hear from a photographer
At a wedding the key issue is getting the content right rather than artistic merit. Getting the ladies without double chins ranks higher than cherry blossom in the background.
Pictures are unlikely to be seen larger than 16x10 so focussing on extreme IQ is wasted time and money.