If you want to diff two files with the same name in the source and destination directory you can save yourself the hassle of selecting the destination file by making a button like this:
"C:\program files\WinMerge\WinMergeU.exe" "{filepath$}" "{destpath$}\{file$}"
You don't normally have to include quotes around the {} arguments but in this case they seem to prevent a quote appearing in the wrong place.
You can also automatically select all files in the destination with the same names as those selected in the source by running:
Select SOURCETODEST
That command is also very useful for quickly checking which files two folders have in common without having to go to the synchronize tool in Opus.