A cute thought but one that ignores the actual realities of the politics involved.
Firstly, remember that it was not the US that declared war on Germany, rather, Germany declared war on the US in tandem with Japan. This means, immediately, that your "play both sides against each other" plan fails from the word "go". Now, could we not have given ass much lend-lease to the Soviets in order to let the Germans do more damage to them? Yes certainly, but there could not be a point in time where the US could switch sides and play them both against each other.
Now you might object we should have been nicer to Japan to prevent them from declaring war on the US. Many folks have their weird idea that somehow the US could have appeased Japan by waving a hand and keeping trade open? Beyond the fact that the Japanese were clearly an aggressive evil empire that no Christian nation should be trading with given the already committed atrocities before their attack on Pearl Harbor that warranted the US sanctions, there was also the simple fact that the US and Japan were pretty much GOING to have a war sometime in the mid-20th century simply due to the rise of Japanese nationalism and their desire to dominate the western Pacific. The US simply had to many colonies, protectorates, trade interests, and allies in the region to simply hand things over to Japan, as well as a vested interesting in maintaining a strong naval position in the central and western Pacific. While yes, theoretically the US could have remained neutral in a war in Europe in the mid-20th century, War in the Pacific was pretty close to inevitable as it was an old fashioned conflicting spheres of power conflict between two empires where one was going to end up dominating over the other, and once that blew up the Germans decided to get in on the action too, and before you object and say "the German declaration meant nothing, Germany couldn't do anything to threaten or harm the US" while you're to a degree correct about the mainland, they did have a significant submarine force that would be engaging in unrestricted operations against American shipping. Thus they could harm US citizens, property, and interests, albeit pinpricks, but also enough that it would turn popular sentiment against the Germans that would further prevent playing the Soviets and Germans against each other.