If two companies agree not to sell a product for more than 20 dollars, have they broken any laws

The answer is "it depends." As a general matter, antitrust laws prohibit certain agreements on price between competitors. However, the purpose of such laws is to promote competition, and an agreement on a maximum price (rather than a minimum price) is generally pro-competition. Also, it matters where the two companies are in the supply chain. Generally, resale price maintenance agreements are permissible.