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Plant them both.  The one that has sprouted will be fine.  If the sprouts are very long and have leaves, plant it so that the leaves are slightly above ground, but invert a flower pot (one that has a hole in the bottom) over the leaves, for four or fives days so that the leaves do not sunburn.  The other tuber may not have survived the winter, but plant it anyway just in case it still has some life in it.  It seems that one of the tubers had enough strength to survive over the winter!   To help the tubers gain the strength to get through the winter, cut down the foliage in the fall when the foliage has yellowed, not before.  Even if there has been a light frost the tuber will be fine.  Lift carefully, wash the tuber and allow to dry in a frost free place with good air circulation.  When the tuber exterior is dry, place in a plastic box, cardboard or wood container with some peat moss or vermiculite to keep the tubers separate.   A wood or cardboard box will allow too much moisture to escape the peat moss covered tubers unless you line the cardboard box with plastic wrap.  This will keep the necessary moisture in the tubers.  The basic idea is to allow the tubers to store maximum food and water in the fall, wash to clean  the tuber of any soil borne organisms, dry off the exterior of the tuber and store in a frost free place, separated by peat moss, in a container that will hold the remaining tuber moisture.