If you could enter/exit Zombie multiple times you could potentially have up to 30s of being unable to die from direct damage (enter Zombie as Living dead is about to wear and the effect would last almost 10s after Living Dead is over). While that would be difficult to pull off and of arguable usefulness, I don't see SE designing the ability that way.
Makes a lot more sense for the Living Dead buff to be traded for the Zombie effect, puts it pretty much perfectly in line between HG and Holmgang.