I would recommend to simply put the semi-transparent 3D rendering in front of the slice view, but for example you can make the red slice view transparent by copy-pasting this to the Python console (before showing the slice view in 3D):
getNode("Red Volume Slice").GetDisplayNode().SetOpacity(0.5)
See some discussion about this feature here: Can the image slices be made semi-transparent in 3D view?