While the US was fighting on two fronts, its enemies were also fighting on multiple fronts. Germany fought on two fronts for most of the war, increasing to three after the invasion of Normandy. Japan was fighting on three fronts for much of the war (China, Burma, the Pacific), increasing to four at the end of the war with the Soviet invasion of Manchuria. The US was also not fighting alone, but alongside allies who also had large armed forces (Soviet Union, Britain, China). At peak strength during the war, the US armed forces were larger than the German armed forces, and much larger than the Japanese armed forces. The Soviet armed forces were similar larger than both the German and Japanese. Add to these the British and Dominion forces, and the Chinese armed forces, and Germany, Japan, and Italy were greatly outnumbered.

Where Germany and Japan thought than the US was weak was in its willingness to accept very high casualties. The Soviet Union and China took very high casualties (many millions), and kept fighting as a matter of national survival. The survival of the USA was not at stake, and the assumption was that similar casualties would cause the USA to pull out of the war. Given that neither Germany nor Japan managed to inflict such casualties on the US armed forces means that we don't know the truth of this assumption.

If the US was fighting alone against both Japan and Germany, it would have been far more difficult. If the US was fighting a defensive war against them, they might still have won: US industry was formidable, and the US population was close to the German and Japanese populations combined. If the US for some reason invaded both Japan and Germany, they might well have lost.