>For a startup it is vital to not have half of your companys knowledge walk out the door.
This strikes me as putting the onus on the company to ensure that knowledge isn't monopolized (as a previous commentor suggests) and, perhaps more importantly, to ensure retention of key people.
One way is to provide ownership, perhaps in the case of a startup, literally, of the company and its outcome.
In effect, this is the difference between another (likely unequal share) co-founder and merely an employee. Even if such a co-founder moves on, there's a strong enough incentive not just to "walk out" and leave the company without access to that knowledge.