Cover doesn't stack either; it just upgrades to Superior Cover if three or more of the "imaginary lines" are obstructed. (PHB pg. 280)



Dmitri the peasant wants to fire on this hobgoblin with his sling. Dmitri's player draws an imaginary line from a corner of his space to each of the four corners of the hobgoblin's space. One or two of the lines are obstructed; the hobgoblin has cover against Dmitri's sling attack.



An alternate-universe Dmitri is fighting a hobgoblin and a skeleton at the same time. In this case, due to the skeleton 3 of Dmitri's imaginary lines are obstructed. The hobgoblin has superior cover against Dmitri's sling attack.



Dmitri from Universe A repositions. Despite the lack of skeleton, the hobgoblin still has superior cover because at least 3 of the imaginary lines are obstructed.



The hobgoblin calls to Maglubiyet for aid, and a big-ass statue falls out of the sky, blocking Dmitri's line of sight. However, despite the hobgoblin deriving cover from two sources, he only benefits from ordinary cover; only two of the imaginary lines are blocked.



Suppose Maglubiyet drops the statue in a slighter better position. Now the hobgoblin is getting two sources of cover, and gains superior cover thereby — three of Dmitri's imaginary lines are obstructed.

CONSIDER YOURSELF EDUCATED.

What, you don't want my bikini-clad body?