Under no circumstances would I ever be in a league where someone owned multiple teams. Even seemingly even trades are still implicitly aiding one of his/her teams over another.
I offer plenty of reasonable trades that are turned down, simply because some owners do not like to trade, or they have a connection to the player I am interested in. Early in the seaon, one league owner would not trade me Ian Kinsler for Big Papi and Brandon Beachy (before both were injured), simply because his 10 year old son was a huge Kinsler fan.
Reasonable trades are rejected all the time... and if one owner has two teams, then you have removed that possibility from the equation and that by itself is an unfair advantage.