when ever i solo i get decent players. but the second i team up i always get the either the trolls, or the toxic as hell players. why ? does Riot have a built in hatred of two friends trying to have a good time?
Riot looks at how good you and your duo are individually, then they add some MMR to that. The idea is that since you practice together and can talk to each other that you'll do better than Solo players, so the MMR boost offsets that.
The reality is that you almost always feel like the enemy team is better than your team, and it does become significantly harder to duo. I had to stop duoing with my wife in Ranked because we just couldn't overcome the MMR difference and it just made us angry.
At some point in 2019 they'll implement a new system that keeps track of your win rate with specific duos, adjusting that MMR to a level that makes sense for you. That should come out when individual lane ranks come out.
