Yolk is a noun and can never be a verb. Yolk is the yellow part of the egg. It is the only ever noun.
Example: He spilt all the yolk on his friend’s t-shirt.

Yoke can be both noun and a verb. Yoke as a noun is a wooden tool used to join two livestock at the neck so that they can work together. As a verb yoke describes the act of k=joining things together
Example: You have to yoke the oxen to put them to work.