Questions before installing multi store

We now have several seperate stores. We want some stores connecties with one shopping cart and one checkout. Therefor the multi store is fine. Stores all have there own database and their own domain. Does the multi store works when we keep the databases or do we need to import the second database from the second shop in the main database ?
Extra info : approx 2500 products per store

Something like linked below could be of help on such, but
probably not for latest Versions ...
Ernie
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Multisite with different databases and images
https://www.opencart.com/index.php?rout ... n_id=24666
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Store Sync - create MultiStores!
https://www.opencart.com/index.php?rout ... n_id=18460
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