The only realistic way for a lake to have more than one out-flowing river is for those rivers to be man made canals. Even if two rivers flow out at the exact same height, exact same slope, and over the exact same rock this will only be temporary. One happy accident and a rock cracks and allows a tiny bit more water to flow through it and the other river will dry up. If you'd like to experiment then dig up some dirt and make a little hill, dig out the center to form a bowl and pour some water in. You will see some of the water flow over the top but you might also get one of the sides of the hill to collapse. Larger lakes don't run as fast as the little example but they act similarly nonetheless. Plus, it's fun to just play in the dirt from time to time.