I'm hoping that we can share/co-author a RH project by uploading it to OneDrive. Is that possible?
If you mean the new UI version of 2019, you can share via the cloud. It's covered in the sample project RoboHelp Reimagined. Without version control though, the practice needs discipline. You can't both work on the same topic or on things like the TOC.
Backup regularly and proceed with caution.