windmove-window-distance-delta

windmove-window-distance-delta is a variable defined in `windmove.el'.
Its value is

1

Documentation:
How far away from the current window to look for an adjacent window.
Measured in characters either horizontally or vertically; setting this
to a value larger than 1 may be useful in getting around window-
placement bugs in old versions of Emacs.

You can customize this variable.