Having the same issue on FMP 13, this function Mirror Layout simply does not exists!
Please help us find this crucial function.
TSGal has answered this issue on the bug-report Help! no Mirror Layout function exist on FM Pro 13:
Modified versions of FileMaker Pro by WinSoft have this ability in FileMaker Pro 12. You will have to check with WinSoft directly to see if they added Mirror Layout function for FileMaker Pro 13. Go to the WinSoft web site at:
Interesting that such a function existed/exists!
Good Luck :)