File comparer where files can be edited during compare

Question

Do you know any file merging too, where meantime I can edit files? I do not want save and reload just edit end merging would automatically reloaded.

I tried FileMerge and Kaleidoscope. None of them has this feature. For huge files it would be useful if about chunk of code merger does not realize Thea are the same, some character is changed, and I would just correct it right at that point.