There is only one file type that uses the .MERLIN2 extension:
Merlin Project File
A MERLIN2 file is a project plan created in ProjectWizards Merlin, a project management application for macOS. It contains project resource, task, and timeline data used to show a project plan in Merlin. .MPROJECT files replaced MERLIN2 files when Merlin Project and Merlin Project Express replaced Merlin.
If you still have access to ProjectWizards Merlin (Mac), you can open a MERLIN2 file in Merlin. Otherwise, you can open a MERLIN2 file in Merlin Project (Mac, iOS) or Merlin Project Express (Mac).
Updated: May 7, 2021