From the documentation:
dired-dwim-target is a variable defined in `dired.el’. Its value is nil
Documentation: If non-nil, Dired tries to guess a default target directory. This means: if there is a Dired buffer displayed in the next window, use its current directory, instead of this Dired buffer’s current directory.
The target is used in the prompt for file copy, rename etc.
Put this in your init file:
(setq dired-dwim-target t). Then, go to dired, split your window,
split-window-vertically & go to another dired directory. When you will press C to copy, the other dir in the split pane will be default destination.