Total Commander

It is possible to view left and right files by pressing F3 and Shift+F3 correspondingly.
Quite logical would be to allow editing files in conifgured editor using F4 and Shift+F4.
Currently these keys have no function in Sync tool.

As a workaround:
In the Synchronize tool you can compare two files with Ctrl+F3 (or doubleclick), and in Compare you can edit one or both files...

This would not call the configured editor, thus it would not be a workaround, because the idea was to call a more powerful editor.

A workaround might be an ability to press F4 in file viewer for editing, as could be done in Far manager.
But TC's Lister does not provide such feature.