In global terms, from the perspective of Britain and France, it would have been a very good thing to divide the US in half.
Both those empires would have breathed a sigh of relief, because by 1860 the entire US already had the largest economy in the world, but separately the North and South didn’t. The South would have needed to buy a huge amount of manufactured goods from the North, so there might have been some kind of agreement between the two, although the unpleasant war would have left the South turning towards European manufacturers, pursuing trade agreements with European nations, sooner than it would have turned to the North. In 1860, while the South was rich and productive, it was apparent that the development path the North was on – towards more intensive industrial and urban development – was the recipe for future success.