You can't.
Unison works by comparing both files with their last synchronized version. If both have changed, and the end result is not identical, they are marked as a conflict. Although in that case a merge program can be invoked to resolve the conflict, the modification time plays no role at all in the whole process.
From the manual:
Note that whether a path is updated has nothing to do with its last
modification time—Unison considers only the contents when determining
whether an update has occurred. This means that touching a file
without changing its contents will not be recognized as an update. A
file can even be changed several times and then changed back to its
original contents; as long as Unison is only run at the end of this
process, no update will be recognized.