one camera is not enough - you need two - at least
the 5D is widely considered to be best for portrait and landscape work while the 1D is more sports orientated.
Doesn't mean you can't use a 1D for wedding though, the 1.3x crop factor will impact on the wider end of zoom lenses and that's need to be considered if operating in tight spaces.
I'm pretty sure there's more to it than that though!
A lot of people seem to use the Canon 5D Mk I for wedding photography but I was wondering if the Canon 1Ds Mk II would be good enough and if not why?

10 years ago, a Nikon D1 was "good enough"
Not for weddings, it wasn't - it was Shi'ite - I still used 35mm and 120 film (and a Nikon Coolscan if they wanted digital copies) in preference as the skin tones from the D1 were atrocious...
I only started shooting weddings on digital when the D1x arrived and that was still pretty iffy...:shake:
Canons of that era had much better colour rendition, which is one reason why so many photographers chose them...
Good battery-life - that's rich...lol
...Clearly you thought a D1x was OK enough to get out of the kitbag. But anyone saying Im going to shoot a wedding with a D40 or a D1x would (today) be branded all sorts of things...

very little between them, at normal ISO's you will be splitting hairs between them
5D is better at high ISO (but do you really shoot there?), has a lighter battery / charger and of course has ETTL-II flash. On the downside it is not weatherproof
TBH, investment is possibly better served with better RAW conversion
5D is better at high ISO (but do you really shoot there?)
There is only thing you need from a camera at a wedding:
Reliability.
1Ds MKII is a better camera and a more expensive one.
Between the 5D and the 1Ds MKI I would choose the 5D every day.
I used a D1x for a few weddings, but it took a lot of P&P to get the images looking as good as a 10x12" lustre print from 35mm Fuji Reala.
How do you find the Ni-MH battery of the 1Ds MkII? Do you reckon a 1Ds MkI could stand up against a 5D?
Cheers