Hmm, so you have two independent data sets. I believe what should happen if you choose to sync them is that all the data will be merged together. You should end up with one data set with all the data combined.
Having said that, I do recommend a backup.
Here is how to make a manual backup on macOS:
- Click on Finder, and choose Go > Go to Folder
- Enter ~/Library/Containers
- Find com.momenta.agenda.macos
- Select it and choose Edit > Copy
- Go to your backup location, select the folder, and choose Edit > Paste
On iOS, at the very least, go into your iCloud settings in Settings app, and make sure you force it to make a backup there.
To make a manual backup on iOS is more difficult, but you could use a tool like iExplorer. You can access your Agenda data there, and drag it to a Mac.