This article explains how Backbone handles concurrent edits – when an issue is edited in both synced projects at the same time.
If it happens that the corresponding issues in both projects have been modified, Backbone will identify which change is newer and will only accept the latest change. This way you experience the same behavior as you would if you worked on the same issue in the same project.
By activating the Advanced Settings sync.responsibilityConfiguration and sync.automaticConflictResolution, you can enable a more pessimistic locking approach. Using this mechanism, only one side is allowed to make changes - and not both sides.