One account, separate switches. Worried about save data while transferring to switch two.

So, years ago my sibling attempted to link our accounts together so we could share NSO through our separate switches. Obviously that didn't work, and just combined our accounts into one, transferring all of my save data from games into their account. It hasn't been much of a problem until now. Neither of us have a switch 2 yet, but are worried about when we both get one, and the save data that we have under the account. We are worried that if one of us transfers the account data to the switch two it will delete all the data on the other end, since we are on separate switches.

If one of us transfers the account data to the switch two, will the other get to keep the data they have on their end? Or, will the one that didn't transfer lose everything?