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A bifurcation lake is a lake that naturally has outflows into two different drainage basins and thus the drainage divide cannot be defined exactly because it is situated in the middle of the lake.
Examples
Two nearby lakes in Finland are Vesijako (the name Vesijako actually means "drainage divide") and Lummene in the Finnish Lakeland both drain in two directions: into the Kymijoki basin that drains into the Gulf of Finland and into the Kokemäenjoki basin that drains into the Gulf of Bothnia.
Similarly the lakes Isojärvi and Inhottu in the Karvianjoki basin in the Satakunta region in western Finland both have two outlets: from Inhottu the waters flow into the Gulf of Bothnia through Eteläjoki in Pori and into lake Isojärvi through Pomarkunjoki River. From lake Isojärvi the waters flow to Gulf of bothnia through Pohjajoki river in Pori and through Merikarvianjoki river in Merikarvia. In Karvianjoki basin there has formerly been also two other bifurcations which however no longer exist due to human action.
Lake Diefenbaker in Saskatchewan is a reservoir created by damming South Saskatchewan River and Qu'Appelle River. The lake continues to drain into the two rivers, however the Qu'Appelle receives a much enlarged flow (in essence, a diversion of flow from the South Saskatchewan) due to the damming. Both rivers eventually drain into Hudson Bay via Lake Winnipeg and the Nelson River.
Isa Lake in Yellowstone National Park is believed to be the only natural bifurcated lake in the world to drain into two oceans. Its western drainage is to the Atlantic Ocean via the Firehole River, while its eastern drainage is to the Pacific Ocean via the Lewis River.
Also located in Saskatchewan is Wollaston Lake, which is the source of Fond du Lac River draining into the Arctic Ocean and of Cochrane River draining into Hudson Bay and the Atlantic Ocean.