By default, ContentSync will pull content from all of your spreadsheet sheets. Any new content pushed to your spreadsheet will default to the main sheet.
You can specify which sheet a layer is synced to within the ContentSync Layer Inspector. Look for the sheet customization icon:
Customize Sheet Icon
Once toggled, the sheet customization field will appear and you can customize the name of the sheet you want to sync to:
Now, when synced, the content will appear on your customized sheet.
Content Keys should only appear on one sheet per content version. If you have the content key in multiple places the once chosen to sync is not guaranteed.