Create a manual save then test if you can load it by exiting and re-entering the game page and clicking "Load Manual Save", then choosing the save file.
If you can successfully load it, then you can exit and re-enter one more time and start a new game by clicking "I understand". You can load the old save whenever you like without losing it. The most recent save game will claim the auto save.
If you can not successfully load a manual save, then you will not be able to have multiple save files