Hi, I share my experience as a Lamictal and Keppra user (twice daily).
I took ONLY Lamictal before, it was good for a while, I felt good physically, but increased the dose to stop auras, and I noticed mentally my mind was in a terrible 'fog'. It was very hard to remember things, like words and if verbally asked to do something it just slipped from my memory all the time. I didn't like it.
So I switched to Keppra ONLY, it was good that it relieved the mental fog.