You can use one data source for ID sync, and can later upload the files straight away to this data source using the normal inbound files:
If you have another data source, that is offline and you are not using it's data source ID/ integration code in ID sync. Then you can link these two data sources by uploading an offline ID sync file. It will be an ID match file. Then you can upload the data for that data source too.
Check this doc for more details on onboarding an ID sync file: