There are two flaws with that logic. First most people do not claim that God is absolutely omniscient nor absolutely omnipotent. So go ahead and disprove a God that most people do not even believe in.
Second there are limits to how well logic makes sense. It just might be that when you are talking about God that as logical as you might be that you just can't grasp God.
For example, if there were a hotel with an infinite number of rooms and it was all booked up (full). It could still accommodate a new guest. there are two solutions: 1) logic is wrong, or 2) infinity does not exist. The problem with answer number two is that other paradoxes and math problems require that infinity does exist.
This is the situation that your paradox of the omniscient and omnipotent God not being possible comes up against. Your logic might be flawless but it might still be wrong. or it might be that that omni-God might not be possible but it just might exist anyway.
Here is food for thought:
http://en.wikipedia.org/wiki/Omnipotence_paradox
Here are some more paradoxes that are unresolved. WE might not understand them, they might be false (illogical) or logic might have limits.
http://www.nationmaster.com/encyclopedia/Unexpected-hanging-paradox
And my favorite that probably proves that logic itself is limited:
"This sentence is false."
http://www.iep.utm.edu/p/par-liar.htm
