2v1s are natural and should be encouraged as much as possible. when someone whines the party of 2 should say something like,

"you snooze you lose"
"welcome to the jungle"
"pwned"
"sorry i wasnt reading chat"

following the successful 2v1 the players should immediately truce or continue the newly formed silent truce with the intention of letting the other player weaken himself. you can kill them later after you have stacked.

In all scenerios people responding to the 2v1 with "hey no 2v1!" are just jealous and want to be part of an even better 3v1 (of which i wll post later)