I think you can show your twenty as soon as the other person forgets to show theirs, and yours counts. Mind you it's been awhile since I played. 

 Scenario: While playing a foursome, my partner and I each announce twenties before play starts. Before second cards are played, it is determined my partners twenty is better then mine so ... 

 I think they should add a small tax on all Motel and Hotel rooms sold in Cape Breton and save the HAT