Well first, Germany wasn't fighting on two fronts, they had captured all of Western Europe except for England. Second, Germany didn't invade during the winter. The plan was to crush the Russian armies decisively during the spring and summer and be done before winter. Germany was supplying it's war effort by getting the resources it needed from Russia in exchange for manufactured equipment. As the war progressed, Germany began to fall behind on their end of the deal, and Russia began to threaten Germany with cutting off their supply of resources. This was one of the big reasons for Hitler's decision to attack Russia as early as possible. However, the Italians were making a mess of things in southern Europe, and Germany was forced to divert their forces to clean up the mess Italy made. This resulted in the German offensive against Russia starting 6 weeks later than originally planned by the German High Command. The end result was the German army beginning to encircle Moscow just as the first blizzards of the Russian winter set in at the start of December. Had they started the offensive in May as originally planned, it's quite possible that the German army would have captured Moscow and the additional 500,000 new Russian troops being trained there. If that had happened, the Russians wouldn't have had any troops left on the western front with which to start a counterattack.