Typically you need 33 HCP to bid a slam with two balanced hands because you don't have long suits to give you extra tricks. If you know you have 33 points between the two hands you can just bid 6NT, but if you are close to 33, you can use the quantitative 4NT. Partner will either pass with a minimum opener or bid 6NT with a good opener.
North opens 2NT to show 20-22 points and a balanced hand. South invites to slam with 4NT but North has a minimum 20 points and so passes.
South's hand is the same but this time North has 22 points and is maximum for the 2NT opening. 6NT makes 6.