My answer shouldn't be treated as definitive, but I did google around a bit.
One youtuber claims that he can access books from two accounts by logging off the device with his original account, logging on with the second account, downloading the ebooks locally, logging off again and then relogging with the original account.
Youtube commenters are skeptical -- and so am I.
You can try, but you might screw up one or both accounts in the process.
Amazon has a method to link ebooks within different accounts within a family -- but there are strict rules about eligibility, plus I highly doubt it would apply to accounts for amazons from different countries. https://www.amazon.com/gp/help/customer/display.html?nodeId=201733400
In my opinion (I'm speculating here) one solution might involve having two separate instances of an app running on a device -- one with each account. I don't think it's feasible though on either android or ios or native kindles.
Perhaps if you could find the unencrypted versions of the files you could transfer them to a second app capable of reading .mobi files. (I know you asked a question about that a little while ago). Perhaps you can use an app like fbreader to read unencrypted .mobi files . https://fbreader.org/content/book-formats-supported-and-not-supported
It seems easier just to have two separate devices with different accounts registered on each one.
Here's a wild idea -- why not write the Amazon people and ask them if they might consider transferring ownership of the files from one account to the other -- especially if you can demonstrate that you own both of them. I'm guessing that you aren't the ONLY Amazon.com customer who has moved to a different country. After all, you clearly have bought the ebooks in question. (Don't keep your hopes up!