You won’t be able to do it through a button in the Wix Editor as that will always be submitting to the one dataset.
You will have to use code to do it and you can see Wix Data API and the save, bulk save, insert, bulk insert, update and bulk update functions here. https://www.wix.com/corvid/reference/wix-data.html
https://www.wix.com/corvid/forum/community-discussion/write-in-multiple-datasets-with-the-same-input
https://www.wix.com/corvid/forum/community-discussion/submit-save-data-entries-into-2-differents-database-issue
https://www.wix.com/corvid/forum/community-discussion/how-to-insert-a-data-in-two-collection-from-a-form
https://www.wix.com/corvid/forum/community-feature-request/submit-to-two-datasets-via-one-submit-button