There is plenty of recent evidence regarding the tight interaction and functional cohesion between the two economies of the United States and China.
This is a peculiar case of economic integration, to which one is inclined to apply a special term — "twinning" because the partners with their outstanding economic potentials do not belong to one economic region and are divided by a gulf of social and political differences.
However, an attentive eye can find other "Siamese twins" in the modern world, though not so mighty and unique as the "pair" described above but also viable and competitive enough.