The LinkedCustomFields plugin allows you to link values between two custom fields.
There is no formal release for now, but the latest version from the repository is expected to be stable.
The functionality is accessed from the 'Manage' menu, where a new item named 'Configure custom field links' is present. At the moment it is only possible to filter for ENUMERATION and MULTI_SELECT custom field types.
When editing a link, each value from the source field can be linked to any number of values from the target field. Selecting no values means that all values are valid.
To report an issue or feature request for LinkedCustomFields plugin, visit Mantis BugTracker, Plugin - LinkedCustomFields.
LinkedCustomFields plugin is hosted at GitHub along with other plugins at https://github.com/mantisbt-plugins/linked-custom-fields .