The shape of the prime focal area will be the shape of the dish, as you go off center with the signal the focal area can become distorted or oval shaped as you are now reflecting the signals from one side of the dish, it is not designed to do this.
Another thing is that a prime focus parabolic dish is designed to put all signals it receives (from one direction) into the prime signal focal area, you mount a LNB outside this area and your signal into it will drop.
Also to receive two (or more) satellites on a prime focus dish you have to offset the center of alignment, if the two satellites are 15 degrees apart then the dish must be offset by 7.5 degrees or your going to have a drop in signal on one of the satellites..
This is where an elliptical dish comes in handy, it can receive two or more satellites with a wider receive beamwidth.
One other problem with useing a prime focus dish for more then one satellite, is that your putting more hardware out in front of the dish, this can have problems with signal scatter, also some prime focus dishes cant be skewed to properly align the LNB's to the correct H and V axis, you have to use a bracket with a built in skew or one with an adjustable collar on it, this adds hardware out in front, go good.
So in my opinion the best dish for more then one satellite would be an offset elliptical, (gets the LNB's and mounting hardware out of the way) you will have a much better chance useing more then one LNB for more then one satellite.