In fact it is very easy to use both iAd and Admob in the same application!
You can find a detailed explanation at this site:
Mixing iAd and AdMob
But in-short the idea behind it is to:
Create an iAd banner and an Admob banner in viewDidLoad
Only the iAd banner gets registered
When iAd fails Admob banner gets registered
If iAd would become active again, the Admob banner will disappear
It even uses a sliding effect to hide and show the advertisements in the bottom of the screen.
So far I can't give a correct number on the revenue, but I can say that the fill rate for iAd is around 30% and for Admob around 95%
Hope this helps you out!
Regards, Luc