You can get multiple outlets if its a temporary measure like damming or if there is a sudden rainfall causing temporary flooding. You can get it to some extent with glacial action but it would have to be pretty active or perhaps seasonal whereby, for example, one river is dammed through being iced up for only parts of the year. Otherwise, in general and in the longer term, one river will always win since its an obvious note that you had two rivers with good carrying capacity and it was steady flow then the water in the lake would fall to the lowest. Where they are equal, one would silt up before the other and then the winner takes both flows thus increasing the erosion on that flow and causing sediment build up on the other. So a man made or changing environment I could believe but I don't think it can happen in a steady state long term deal.