Splitscreen effects are now possible by allowing Multiple Viewports. To use them, you need to attach a python script to a second or third (or numerous) cameras:

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from Rasterizer import *

own = GameLogic.getCurrentController().getOwner()

own.enableViewport(1)

own.setViewport(0, getWindowHeight()/2, getWindowWidth()/2, getWindowHeight())

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You can also have views that are embedded over the top of another view. See the example blend for code (note that the example is incomplete since the embeded window needs to be drawn last...).