When you go to propose a merge there are 5 options ( I *think* everyone can see all 5 but I might be wrong).
Compare, Merge, Match, Reject, Remove.
You can enter the two ID numbers like you would to propose a merge but then click Remove instead. It'll take you through another screen to confirm and then it will state that any connection between the two profiles has been removed.
After that it is unstuck and can be proposed and completed as normal.