As Pampas is a grass it is best to split it during the spring as it is begining to show new growth.
To be honest the only time I have split a Pampas is to get rid of it because it got very large and I wanted the space for other more interesting plants. Any small bits of root/shoot that I happened to leave in the ground managed to grow so it is potentially very easy to divide. although in reallity because the clump is so tough it ends up being hard to divide. Try and cut a bit from the edge of the clump but beware the leaves will cut your hands.
I used an axe a pick and a spade to get rid of mine and afterwards I was k*******.
