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Who ever writes last will overwrite the first entry. It is very unlikely that you will have issues as the users would literally have to change the same value within a refresh cycle of each other. Its acutally a selling point that multiple users can be in the same schedule.
Also, a message will appear if you try and schedule a project someone else has open. I would instruct them that if that happens they need to double check their work. They should be regularly hitting F5 to refresh their data.
If you're having only two users and both don't need to work in the same project to the same time then you can advice them to use open exclusive instead of shared mode for opening the project.