The US could manage a two front war because of the nature of the two wars it was fighting and the fact that it was, and is, a rare exception in warfare: A sea power and a land power. The previous example of such a nation was Rome in the Punic Wars. Anyway, for the Pacific War the US needed sea power and put most of their fleet there. In the Atlantic, they were facing just the German Guerre de Course of merchant raiding by U-boats. With the added advantage of having Britain, a sea power, aligned with them the naval wars the Allies faced were manageable. The US also produced a very large land army and equipped numerous allies to a high standard. This meant they had the land power to fight a land power, Germany. Of course, in terms of economics, the Germans were pretty much hit having a mere fraction of the economic power the Allies possessed. It is of interest to note that even in the late 30's as the US began to seriously invest in preparing for war they were virtually outspending the Germans who were pushing the limit of what they could economically afford.