Explanation from Riot or maybe knowledgeable player about a certain Mastery - Assassin

I just want to know how it works. I know it works in a 1vs1 situation, but how about 2vs1 or 1vs2, is one of those going to work or does it only work in 1vs1? Not sure by the description which way it works but I assume it works as 1vs2 and not also 2vs1 since it uses the word 'champion' in the plural form the way it does but want to make sure.