Sorry but what you are trying to do is impossible because they could look alike but they are different blockchains, and in the name blockchain you have the answer why it couldn't be done. You can't make a composed chain taking a piece from somewhere and the compliment from another site.

I know it sucks, but you will have to spend those 200Gb on each chain... there are no shortcuts for this. But the idea is great maybe devs can create a tool to do it.