IS it possible to get a [preview] wiev with half speed?
This may be the first time I've ever heard of the need for a half-speed preview!
Usually, people complain about slower-than-realtime previews.
To my knowledge there isn't any easy way to do it. You could try nesting the current comp inside a new one, then doubling the frame rate of the new one and previewing.
You can manually override the framerate in the preview panel's option. If it's not visible, go to Window --> Preview.