Hi Paul, sorry for the trouble. Balsamiq for Desktop does not yet support real-time collaboration. What you described is unfortunately the expected behavior.
We actually just released a new version of our plugin for Google Drive today, and it does support real-time collaboration, as long as people are working on different mockups. If you are working on the same mockup at the same time, you will run into issues. If you want, you could check it out here: https://chrome.google.com/webstore/detail/balsamiq-mockups-projects/iedapplgopkgngalkbailjoikghljkki.
We are working toward having real-time collaboration in the desktop app as well in the future, but that is a thorny issue. It depends a lot on how the mockup is saved and where you are trying to collaborate. Right now, we don’t support well editing the same document on a shared network drive, or with Dropbox, or anything like that. Hopefully we can get there in the future. No ETA though. Thanks! And sorry I don’t have better news for you!