When publishing books it is good to have your own ISBN. You can use the same ISBN to publish your books on other platforms. In some instances, you might have to fill out a form to transfer a title from one distribution platform to the next.
For example, you can publish the same book with both platforms such as KDP and IngramSpark.
First, you should publish your book with KDP using your own ISBN and then with IngramSpark. While publishing with IngramSpark you will get a message that your ISBN is already in use. You will be given a form to transfer your title from Amazon to IngramSpark. Fill it out and submit it to IngramSpark. For no reason unpublish your books from KDP. You will lose your reviews.
The only thing you must do with KDP is to remove your books from other distribution channels for transfer to happen. Do this as soon as a form submitted to IngramSpark.
Once it happens you still will have your books on KDP and IngramSpark at the same time. So your income will come from both channels.
ISBN can be bought from Bowker. It is cheaper to buy 10 at a time than one.
I must warn the cost of printing books with IngramSpark cost more but they distribute books to many channels and print hardcover books as well. Bookstores prefer buying books from them. Customer service is good but the royalties statement can be confusing. You have to pay a small amount to set up an account and publish your books. After that, you will pay 25 US if you decide to republish an edited book.
If you plan to publish a hardcover book strictly use their book cover template that would be built according to information provided by you. If you do not follow the template's requirements, your book will not be accepted.
Never accept book returns when publishing your books. Those returns will eat up your money.