You can easily copy stuff between vim splits, though. The window movements can also be part of vim macros. I have a couple macros that will assume I'm on the left window, make a change to the right window, save the file, and return me to the left side. I also have one to delete the second line from two side-by-side files for playlist management. At one point I used several vim instances in tmux panes, but only because I hadn't learned of vim's own advanced features.