Does anyone know how to code the bitcoin.conf file to do the above as franky1 understood?
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.
I am glad to come across this discussion today, I long to have higher knowledge on how blockchain work and also become blockchain developer. I have not come across anyone physically in my country, so I will appreciate if I see an online mentor. I will like you to be my mentor if you are a blockchain developer.